Gas Liberado

Embed Size (px)

Citation preview

  • 8/12/2019 Gas Liberado

    1/18

    "LIBERATED, PRODUCED, RECYCLED OR CONTAMINATION?"

    byR.F. Mercer

    Continental Laboratories Inc.Calgary, Alberta, Canada.

    (ABSTRACT)

    ufficient evidence exists to suggest that misinterpretation of wellsite gas detection data is quite common. The questoften asked, "How big a gas show should I expect to get from a zone that will make a well?" Such misunderstandinay often be traced to a lack of familiarity with the fundamental principles of gas detection and interpretation.

    o illustrate these fundamental principles, a drilling model is presented to demonstrate the effects of bit penetrationodel is analyzed to explain theoretical gas detection response to the penetration of a hydrocarbon bearing zone.

    how characteristics, as transmitted to the surface by the drilling fluid, are specifically related to bit penetration. A canalysis for the drilling model derives four

    assifications of gas present in the drilling fluid. These are:

    1. Liberated gas2. Produced gas3. Recycled gas

    4. Contamination gas

    strong case is presented to show that all drilling fluid hydrocarbons may be classified into one of the four categorie

    efinitions are provided for each type of gas.

    1. Liberated gas is defined as gas mechanically liberated by the bit into the drilling fluid as the bit penetrates theformation.

    2. Produced gas is defined as gas produced into the drilling fluid from a specific zone in response to a formationpressure which exceeds the opposing effective hydrostatic pressure.

    3. Recycled gas is defined as gas which has been pumped back down the hole to appear a second time at the surf

    4. Contamination gas is defined as gas artificially introduced to the drilling fluid system from a source other thanrock formations.

    eological and drilling engineering implications of each category are discussed. Abnormal pressure detection andurveillance receive special consideration.

    is concluded that a working knowledge of basic principles of interpretation is absolutely requisite to an effective usellsite gas detection data.

    INTRODUCTION

    he author has noted over the last few years that considerable misunderstanding exists with regard to techniques of

    Pgina 1 d

    19/10/tp://www.continental-labs.ab.ca/lib.htm

  • 8/12/2019 Gas Liberado

    2/18

    terpretation of hydrocarbons found in the drilling fluid during penetration. This paper will seek to provide a basicnderstanding of these principles to assist concerned persons in the efficient use of gas detection data. Theseundamentals should provide the basis for a creative understanding of the mechanics of gas liberation and detection aell as assisting the responsible person to render accurate and timely decisions more efficiently.

    uch information should prove beneficial as applied to a number of different phases of the total drilling and completiperation. It will be shown that mud gas data is of great value in delineating the precise thickness of potential reservo well as providing assistance in identifying the hydrocarbon-water interface.

    enerally speaking, down hole logs accurately measure physical characteristics of the rock in place but do notpecifically measure distinct physical characteristics of hydrocarbons. The presence of hydrocarbons is projected on asis of relative changes in tool response for one measurement compared to other measurements covering the sameterval.

    n contrast, gas detection data measures a specific physical characteristic of hydrocarbons and consequently respondsrectly to their presence. The gas detector is not capable, however, of giving definitive data regarding important phy

    haracteristics of the rock. The interdependence of gas detection data and down hole logs should therefore be readilypparent.

    will also be shown that intervals of hydrocarbon potential can, in many instances, be identified with greater resolan is available from other sources. Such information has proven to be of valuable assistance to operators requiri

    ery selective perforation program.

    he presence of hydrocarbons in the mud system is shown to be an important indicator with respect to abnormal presetection. Continuous surveillance is essential to the maintenance of high standards of blow out protection and generg safety. In addition to basic safety consideration, gas detection data has proven useful to drilling engineers inetermining the precise mud density requirement to ensure adequate blow out protection while maintaining maximumenetration.

    . GAS LOGGING TECHNIQUE

    efore a detailed consideration of gas detection interpretation is presented, a short background may prove helpful to

    ose not directly familiar with gas logging techniques.

    he basic function of wellsite hydrocarbon detection has been defined in a previous paper as follows: "to detect andosition with respect to depth all hydrocarbon accumulations." (1)

    his definition relates specifically to techniques employed at the wellsite to determine potential hydrocarbon bearingones. Both drilling fluid and cuttings detection are included. A significant increase in the concentration of hydrocarb

    the mud system is referred to as a "gas show".

    n contrast, wellsite gas analysis is performed by chromatography to define the composition of "detected" hydrocarbo an aid in pre-determining the character of the hydrocarbons in a reservoir as well as assisting in the proper

    assification of those hydrocarbons contained in the mud system. The primary function of wellsite gas analysis has aeen defined in the earlier paper as follows: "to analyze each hydrocarbon accumulation to include the identity ahe relative proportion of each component". (2)

    ypical gas detection and analysis equipment in use throughout the world generally incorporates one of two standardetectors. By far the most popular total gas detector is the catalytic filament detector (CFD). It operates on the princif catalytic combustion of hydrocarbons in the presence of a heated platinum wire at concentration levels below thewer explosive limit. The increasing heat due to combustion causes a corresponding increase in the resistance of theatinum wire filament. This resistance increase is measured through the use of a wheatstone bridge circuit and recor, "units of gas".

    he second detection technique is provided by the flame ionization detector (FID). This detector functions on the

    Pgina 2 d

    19/10/tp://www.continental-labs.ab.ca/lib.htm

  • 8/12/2019 Gas Liberado

    3/18

    rinciple of hydrocarbon molecule ionization in the presence of a very hot hydrogen flame. These ions are subjected rong electrical field resulting in a measurable current flow which is then amplified and recorded. A thorough discusf this unique technique as applied to wellsite gas analysis is available in the literature. (3)

    he total gas detector and the chromatograph are normally installed as companion instruments at the wellsite and procontinuous monitoring of the drilling fluid and well cuttings for the presence and composition of hydrocarbons.

    lthough cuttings gas detection and analysis is of considerable value in its own right, it will not be discussed in thisaper.

    nfortunately the term "gas detection" has often proven to be misleading because it appears to suggest that gas detecquipment is only of service in locating gas reservoirs. This is not the case. As shown by Evans, Rogers and Bailey, ature liquid hydrocarbon reservoirs are characterized by rich compositions of all components in the gasoline range rough C7 with a good distribution of gases in the range C2 through C4 plus reasonable quantities of C1. These fact

    emonstrate that gas detection equipment should be more properly called hydrocarbon detection equipment since it iffective in locating both gas and liquid hydrocarbon reservoirs.

    I. A DRILLING MODEL

    equisite to a clear understanding of the interpretation of mud-gas data is consideration of the source of hydrocarboney occur in the drilling mud. To assist in this consideration, a simple drilling model is proposed which illustrates th

    mpact of bit penetration through hydrocarbon accumulations. A series of cases is presented where variations in theonfiguration of the mud-gas data indicate specific differences in the response of the hydrocarbon bearing zone to bitenetration and subsequent rig operations.

    he model will show that the geometry of the gas show recorded by the instrumentation and plotted with respect to tdirectly related to significant characteristics of the hydrocarbon zone as well as the impact of concurrent drilling

    perations. It will become apparent that the configuration of the gas show as recorded directly from the drilling mud reater interpretive significance than the magnitude of the gas show. When instrument chart data recorded versus timgitized and plotted in graph format versus depth, the magnitude of the gas show may be faithfully reproduced but t

    onfiguration of the show is usually lost.

    hus it becomes obvious that basic and vital interpretation must be derived from a detailed analysis of the instrumentharts themselves and not solely from a plotted graph. The basic function of the plotted graph should be to collate,ccording to depth, pertinent data produced from various sources. This graph then provides a broader understanding e hydrocarbon accumulation and a convenient means for future reference.

    o illustrate these concepts, a diagrammatic technique has been employed which graphically relates the gas detsponse plotted versus time to the actual penetration of the rock by the drilling bit through the penetration rate cotted versus depth. This technique allows direct

    omparison of the geometry of the gas response to actual rock penetration.

    Pgina 3 d

    19/10/tp://www.continental-labs.ab.ca/lib.htm

  • 8/12/2019 Gas Liberado

    4/18

    . LIBERATED GAS 1. Full Hole Drilling

    gure one illustrates a typical situation where a bore hole is created through a hydrocarbon bearing zone and the totaottom hole pressure (TBP) is greater than the formation pressure (FP). During penetration, the bit continuouslytroduces to the mud system components of the rock contained in the cylinder defined by the hole size and the thick

    f the interval. As the bit penetrates, it mechanically creates pseudo-permeability and allows material contained in th

    Pgina 4 d

    19/10/tp://www.continental-labs.ab.ca/lib.htm

  • 8/12/2019 Gas Liberado

    5/18

    bsolute pore volume of the cylinder to enter the mud system and be transported to the surface. The term pseudo-ermeability is suggested because the liberating action of the bit is purely mechanical and not directly related to theherent rock permeability.

    the pore volume contains hydrocarbons, it is evident that the hydrocarbons contained in the cylinder of rock will bansported to the surface in various proportions of two possible forms. First, liberated directly into the mud or produto the mud from the cuttings as they are subjected to ever decreasing hydrostatic pressure. Second, retained by the

    uttings chips themselves. It therefore follows that the primary source of hydrocarbons available to the gas detectionquipment under these conditions derives from the cylinder of rock mechanically liberated into the mud system by bi

    ction. This is a type one gas response.

    iberated gas is therefore defined as gas mechanically liberated by the bit into the drilling fluid as the bitenetrates the formation.

    n figure one the penetration rate curve corresponding to the porous interval shows a characteristic drilling break as tt drills though the sandstone. Such drilling breaks are often invaluable in determining the thickness of porous intervhe hypothetical gas detector response shows a typical record of the concentration of hydrocarbons in the mud versume.

    he concentration of liberated hydrocarbons in the mud is primarily a function of the following factors:

    1. Penetration rate2. Absolute pore volume3. Formation pressure

    ther factors are also of concern, such as oil and gas saturations, mud return flow rates and hole size. It is assumed fourposes of our example that these additional factors are not pertinent to our discussion of basic principles but shouldonsidered when evaluating the significance of a particular gas show.

    ubstantial increases in any of the three named factors will normally have a visible effect on the gas detector response normal case, the rate of penetration is the most important single factor in determining the magnitude of the gas shhe effect of penetration rates will be discussed in greater detail later in the paper.

    the bit were penetrating the cylinder of rock at a constant rate, and if the porosity and the formation pressure werexactly constant throughout the interval, it is reasonable to assume that an equilibrium would be established betweenolume of mud circulating through the bit and the volume of gas mechanically liberated to the mud system. Thisupposition is shown diagramatically in figure one but of course rarely exists in reality. The lag time is shown as therculating time commencing with bit contact of the porous interval and terminating with commencement of the gascrease. If the gas response comprises only liberated gas, it is reasonable to conclude that the gas response would beend one lag time after the bit ceases to penetrate hydrocarbon bearing porosity.

    nce the formation pressure is normally constant throughout a single porous interval, it is reasonable to conclude thaariations in the magnitude of the liberated gas reading are related to the remaining two parameters; penetration rate

    orosity. Should the penetration rate be relatively constant, show magnitude variations can often be related directly tck porosity with resolution capability less than one meter.

    gure one, a type one gas response, is the normal case because a margin of safety is always desired when penetratinossible blow out zones. Figure one shows the typical situation where mud filtrate has invaded the porous formationhile wall cake was being deposited on the surface of the hole. Since the pressure differential across the hole - rockterface is positive, it is evident that no additional hydrocarbons beyond those contained in the rock cylinder contribthe liberated gas response.

    n unusual circumstances of high formation permeability, low formation pressure and exceedingly high total bottom hressure, it is possible that mechanically liberated hydrocarbons may be pumped directly into the formation and notturn to the surface. A further variation of this possibility may occur if filtrate invasion immediately preceded the bi

    Pgina 5 d

    19/10/tp://www.continental-labs.ab.ca/lib.htm

  • 8/12/2019 Gas Liberado

    6/18

    esident hydrocarbons may be flushed by the filtrate so that the bit would mechanically liberate only mud filtrate andre-existing hydrocarbons. While these two possibilities occur

    ith extreme rarity, they should be considered in instances where gas shows were normally expected but did not occ

    Three basic principles of interpretation emerge from this discussion of liberated gas.

    1. In a type one gas response (ie. where the formation pressure is less than the total bottom hole pressure) onlymechanically liberated gas forms the show.

    2. The configuration of the gas show is an early indication of the thickness of the liberating interval and possibly quality of the porosity as well as the depth and thickness of the most porous interval.3. The presence of a type one gas response gives no direct indication of the presence or absence of

    permeability. If there is no effective permeability when drilling a hydrocarbon bearing zone, the liberateshow will still occur. If permeability is present and a sufficient hydrostatic overload is carried in the system, the configuration of the liberated gas show will remain relatively the same.

    Coring

    he principle of mechanical liberation through a type one gas response zone should have obvious implications for coecause only a small portion of the normally drilled cylinder of rock is being exposed to mechanical liberation by theore bit. It logically follows that a much smaller quantity of liberated gas is introduced to the mud system per meter oenetration. Often, coring rates are considerably slower than full hole bit penetration rates which would further decree liberated gas quantity per mud volume. These factors usually combine to result in the common phenomenon of

    onsiderably lower gas readings while coring.

    . RECYCLED GAS

    n the event that mud gas is not completely volatilized in the settling pit but is pumped back down the hole, the gasetector may record a second appearance of a pre-existing show. This phenomenon is diagrammed in figure one whee liberated gas show has recycled to the surface for the second time and is designated R.

    ecycled gas is therefore defined as gas which has been pumped back down the hole to appear a second time ahe surface.

    n analysis of the usefulness of recycled gas is available in the literature. (5)

    ecycled gas may be identified by the application of certain tests. The recycle should be no larger than the originalsponse but should be similar in shape. The composition of the recycled response may be misleading in that the mor

    olatile hydrocarbons are often liberated to the atmosphere in the pits and under the influence of a degasser. The resue analysis of the recycled response shows a larger proportion of heavy ends.

    rom the beginning of the primary gas response to the beginning of the recycled gas response in circulating time

    ood indication of the total circulating time of the mud system. Such direct information may often be helpful in assue accuracy of an estimated lag time.

    . PARTIAL LIBERATION

    gure two demonstrates possible alternative explanations for instances where the duration of the gas show does not extend throughout the entire period of probable liberation as projected from other indicators such as the penetrationte.

    Pgina 6 d

    19/10/tp://www.continental-labs.ab.ca/lib.htm

  • 8/12/2019 Gas Liberado

    7/18

    n a type one gas response (TBP>FP) only liberated gas would comprise the gas response.

    the geometry of the show is solved in a manner consistent with the principles derived in figure one, a significantariation within the interval of the drilling break becomes apparent. Two alternative

    xplanations are suggested in (A) or (B) as shown in figure 2.

    A. Since gas was mechanically liberated only from the top portion of the drilling break, it is probable to assume ththe best porosity occurs through that interval. The absolute pore volume is probably diminished or absent throuthe bottom of the section resulting in no liberation. If liberation should occur from the bottom of the zone and nfrom the top, this explanation would be favored over (B) because gas does not naturally occur under water in a

    Pgina 7 d

    19/10/tp://www.continental-labs.ab.ca/lib.htm

  • 8/12/2019 Gas Liberado

    8/18

    contiguous reservoir. In case (A) the constant penetration rate throughout the drilling break would probably refbetter bit performance in sandstone than shale. The distinction between drilling porous and nonporous sandstonappeared to be of little consequence by comparison.

    B. If indications suggest that the porosity does in fact continue throughout the interval as delineated by the drillinbreak, it is probable that the absolute pore volume in the upper section contains only hydrocarbons while the lopore volume is filled with water. This principle can be exceptionally helpful in conjunction with log saturationindications in determining the gas-water interface or transition zone.

    D. EFFECT OF PENETRATION RATE CHANGE ON SHOW MAGNITUDE Figure three again considers type one gas response where only liberated gas is present.

    Pgina 8 d

    19/10/tp://www.continental-labs.ab.ca/lib.htm

  • 8/12/2019 Gas Liberado

    9/18

  • 8/12/2019 Gas Liberado

    10/18

    gure four shows the usual situation where the hole does not begin to make fluid immediately upon penetration oone but the gas response commences at one normal lag time. Such responses are characterized by exceptional iagnitude and the continuation of the response

    eyond the time normally anticipated for the termination of the liberated show.

    the source zone is clearly defined by the penetration rate and other available geological data, it becomes apparent te formation is contributing additional hydrocarbons to the mud system beyond those mechanically liberated.

    roduced gas is therefore defined as gas produced into the drilling fluid from a specific zone in response to armation pressure which exceeds the opposing effective hydrostatic pressure.

    Pgina 10 d

    19/10/tp://www.continental-labs.ab.ca/lib.htm

  • 8/12/2019 Gas Liberado

    11/18

    gnificant contrasts in interpretation result from a type two gas response.

    1. There is now no direct relationship between mechanical liberation and mud circulation, therefore definitanalysis of the source zone thickness and quality becomes extremely difficult. The magnitude of the gasresponse can no longer be related to the general significance of the source zone in comparison with othetype one gas responses.

    2. The presence of produced gas demonstrates conclusively that at least some degree of effective permeabiis present. This direct evidence of permeability is in contrast to the absence of any definitive evidence in type one response where only mechanical liberation occurs.

    3. Since produced gas is generally independent of mechanical liberation and its attendant controlling factoris reasonable to expect that the configuration and magnitude of type two gas responses encountered whilcoring would be generally independent of the mechanical characteristics of the coring operation.

    . CONTAMINATION GAS

    ccasionally drilling operations require the introduction of oil in various forms to provide additional pipe lubricationc. Oil based muds are often used to minimize formation damage through elimination of excessive water loss. Diesee normal oil phase used in inverted oil emulsion muds. Diesel in its natural state does not contain volatile hydrocar

    nd therefore is not disruptive to gas detection equipment. However, diesel is often transported in containers which hreviously carried volatile crudes and may therefore retain some volatile gases. Hydrogen gas is often detected in pipon or associated with the setting action of cement. Occasionally mud additives or various chemical reactions in the

    ill provide other hydrocarbons or combustible gases which may be detectable by wellsite total gas detectors. All ofese examples comprise combustible gas sources which are not indigenous to the rock formations and must be ident

    ccordingly when detailed interpretation is desired.

    ontamination gas is therefore defined as gas artificially introduced to the drilling fluid system from a sourcether than the rock formations.

    fter working around gas detection equipment for some time, rig personnel become aware of what gas sources cadded to the mud to influence gas detector readings. One must of course establish that the gas source was not delibertroduced by a member of the drilling crew.

    t certain times mud conditions are such that the introduction of large volumes of air into the mud system cause "pseas responses". These responses do not reflect increased gas concentration in the mud but rather greater gas trapfficiency when the air-rich mud reaches the surface. This phenomenon may occur after trips when a float is used or elly air introduced during connections. Such pseudo responses are often called "kelly responses", or have a distinctffect on "trip gas responses". Trip gas will be considered in detail later in the paper.

    V. ABNORMAL PRESSURE APPLICATIONS

    gure five portrays a type two gas response and suggests subsequent rig operations which can be used to deal with abnormally pressured interval with due regard to safety and optimized penetration.

    Pgina 11 d

    19/10/tp://www.continental-labs.ab.ca/lib.htm

  • 8/12/2019 Gas Liberado

    12/18

    Pgina 12 d

    19/10/tp://www.continental-labs.ab.ca/lib.htm

  • 8/12/2019 Gas Liberado

    13/18

    A. CONNECTION GAS

    In this hypothetical situation the rig experienced a type two gas response. After continuing circulation for somtime, the magnitude of the readings continued to increase. At that point a decision was made to increase the mudensity which eliminated the produced gas and returned the mud system to the pre-existing background. The tiscale is of course very compressed in this example and does not accurately portray the time span often necessaeliminate large quantities of produced and recycled produced gas. Subsequent to the elimination of produced gconnection was made. Evidence of the connection appears on the gas detector chart as a decrease in the carriedbackground reading where no mud was circulated during the connection. At approximately one lag time after

    circulation was resumed, a response occurred. This response was deemed produced gas as it was related to theconnection and was not liberated from the formation being penetrated one lag time before the response.

    Because there is no evidence of produced gas in the system while circulating, it is apparent that the mud densitplus the annular pressure drop (APD) are sufficient to create a total bottom hole pressure greater than the formpressure. Therefore, the connection gas peak experienced after the first connection subsequent to penetrating thgas zone may be related predominantly to swabbing of the zone rather than to insufficient hydrostatic pressureSwabbing may occur when the kelly is raised for a connection. Because the annular pressure drop is lost duringperiods of no circulation, the bottom hole pressure is equal to the hydrostatic pressure for static mud systems. Tdecrease of bottom hole pressure may be a factor in the magnitude of the connection peak. Since the swabbingeffect is not measurable , it would be difficult to ascertain with any degree of accuracy the significance ofconnection gas peaks which result when bit movement on connections extends above the gas zone.

    On the next connection, however, when it was certain that no bit swabbing occurred, no connection gas peakresulted. This fact suggests that the mud may be too heavy since no produced gas resulted from loss of the annpressure drop.

    The mud density was subsequently reduced until a moderate connection gas occurred with no increase inbackground values while circulating. The formation pressure of the producing zone is bracketed as follows: Thformation pressure is approximately equal to or greater than the hydrostatic pressure, however, the formationpressure is less than the hydrostatic pressure plus the annular pressure drop. Such circumstances represent theoptimum mud density for containing the zone yet providing positive evidence that the mud density is notexcessively high.

    Subsequent reduction in mud density resulted in measurable quantities of produced gas becoming apparent in tmud system during circulation. This fact suggested that the formation pressure was now greater than the totalbottom hole pressure and that the mud density had been reduced too much. The mud density was then increaserestore the ideal condition of moderate connection gas peaks with no evidence of produced gas while circulatin

    B. TRIP GAS

    rip gas is the general term applied to produced gas which characteristically occurs within one lag time after a trip isompleted and circulation has been resumed. Three basic factors influence the presence, location and magnitude of thip gas .

    (1) The loss of the annular pressure drop.

    1. The effect of bit swabbing the entire hole. This effect is influenced to a considerable degree by such factas the speed at which the pipe is tripped out of the hole, variations in hole size, the configuration of a pahole assembly, and tripping out with a full hole core barrel.

    2. The time over which these factors influence the static mud system.

    he basic principles previously discussed with regard to connection gas of course also apply to trips. The mostgnificant difference between trips and connections is the extreme accentuation of these influences during a round trompared to the relatively minor influence of a connection.

    Pgina 13 d

    19/10/tp://www.continental-labs.ab.ca/lib.htm

  • 8/12/2019 Gas Liberado

    14/18

    his accentuation of effect should immediately suggest the seriousness of ensuring absolute control over any previourilled zone exhibiting abnormal pressure characteristics before a trip is attempted. It would be extremely foolish touspend circulation and commence a trip in the midst of a formation gas response without first ascertaining whether as a type one or type two response.

    he rig is never more vulnerable than during a trip out of the hole especially when the hole is not kept full. Mr. A.S.Murray reported to the Canadian Oil Scouts Association that "80% of all blow outs occur during tripping." He noted 80% of blow out problems occur while drilling with insufficient mud density and failure to fill the hole while trippine also observed that "80% of blow outs come from normal pressure zones and occur in wells less than 2500 meters

    eep." He concluded that, "practically all these blow outs could have been prevented because 80% of all blow outs are result of human failure." (6)

    is apparent from the principles of gas detection as previously discussed, that the gas detector is of limited assistancroviding early warning for zones which blow out immediately upon contact. Proven methods of well control have beveloped and should be employed in these cases. The constant drill pipe method is especially helpful in establishingontrol over such a zone until proper mud density can take effect. Statistics have shown, however, that this type of blut zone is the exception rather than the rule. Therefore, careful surveillance of all produced gas indications, especialsulting from tripping, becomes a very important rig safety indicator.

    gure six presents a hypothetical situation where a type two gas response has occurred followed by the elimination oroduced gas through the effect of increased mud density. Subsequently, circulation was terminated for a trip. Upon

    ompletion of the trip, circulation was resumed and a typical trip gas response was received at approximately on lag this response suggested by its configuration that the mud density was sufficient to keep the well under control and nangerous conditions were experienced during the trip.

    Pgina 14 d

    19/10/tp://www.continental-labs.ab.ca/lib.htm

  • 8/12/2019 Gas Liberado

    15/18

    Pgina 15 d

    19/10/tp://www.continental-labs.ab.ca/lib.htm

  • 8/12/2019 Gas Liberado

    16/18

    the trip gas response had occurred in its entirety at a lag time somewhat less than one total lag time, this would inde possibility of a produced gas source at some shallower depth in the hole. Such information derives from the fact te first circulation after a trip gives some indication of the response of various hydrocarbon accumulations presentroughout the length of the uncased hole.

    the trip gas response indicates a very early onset which cannot be attributed to a shallower source, this may indicatxtent of gas swabbing up the hole following the bit on the trip out.

    n the instance of a normal trip gas response we know that the formation pressure is greater than the hydrostatic press

    inus the swabbing effect or no response would have occurred. But other indications suggest that the formation presless than the hydrostatic pressure alone. Exorbitant trip gas peaks and unexplained variations in magnitude shouldause the seriously interested person to ascertain the explanation for this condition. An extended trip gas response afte main peak suggests that the formation has continued to produce after circulation was resumed. This circumstanceould suggest the hole is barely in balance and should be treated with great care during subsequent trips. The formatressure in this case may be greater than the total bottom hole pressure until the swabbed trip gas is circulated out ofystem.

    n occasion, factors will combine to render the gas detection data received when a zone is drilled apparently lessgnificant than it is, especially if no indications of produced gas are seen. The first trip gas response after such a zonay be significant indication of the presence of permeability and shed greater light on the original interpretation.

    . LIMITED PERMEABILITY INDICATIONS

    ccasionally abnormal pressure intervals may be drilled where no indications are received from other wellsite sourceuch as penetration rate and sample examination. The most probable explanation is fracturing. If such a zone isenetrated in the absence of total gas detection equipment, and therefore there is no evidence regarding the presence roduced gas, the rig crew may be unaware of the danger awaiting them on the next trip. In many instances, a gassponse with large proportions of produced gas will occur while drilling. Subsequent attempts to kill the zone, thusiminating the produced gas by raising the mud density may prove unsuccessful. Even after a considerable increase ud density, the produced gas seemingly continues. One must first be certain that the gas is not a continuing build upcycled gas. If the produced gas does in fact exist and persist, the facts would suggest an unusually high pressureterval of very low permeability which has apparently established a flow equilibrium with the circulating mud syste

    function of deliverability rather than formation pressure. Had the deliverability been greater, the zone would probabave blown out immediately upon contact.

    such evidence exists, efforts to completely eliminate all traces of produced gas may be unwarranted.

    n occasion, evidence of produced gas, such as connection gas and trip gas may diminish over extended periods ofrilling with no apparent change in mud characteristics. Such evidence may suggest that the source zone was of limitermeability and has now depleted to a sub-hydrostatic formation pressure.

    ll of these factors indicate the importance of bringing to bear all available gas detection data on the interpretatiopecific hydrocarbon accumulations including the more subtle indications which are observed over long periods of

    urveillance especially on connections and trips.

    . CONCLUSIONS

    his paper has shown that all hydrocarbon indications in the drilling mud system may be classified in one of the fourerived categories. An understanding of basic principles discussed in the paper is necessary for proper classification.ccurate classification of hydrocarbons appearing in the mud system from time to time is a necessary prerequisite foependable data interpretation.

    Further benefits accrued from an understanding of these principles include the following;

    1. The basis is formed for better communication between the logging crew and the wellsite geologist togeth

    Pgina 16 d

    19/10/tp://www.continental-labs.ab.ca/lib.htm

  • 8/12/2019 Gas Liberado

    17/18

    with the drilling personnel.2. Unnecessary or improper drilling activities may be prevented.3. Early decisions with respect to drilled hydrocarbon accumulations may be rendered with due regard for

    safety. Some examples are as follows:

    a. Do not stop circulating for a trip in the middle of a gas show until it has been clearly ascertaif the show is type one or type two.

    b. Mud density increase may be initiated immediately if necessary without losing the annularpressure drop during periods of non-circulation.

    c. Instructions regarding special care during tripping may be given to the drilling crewimmediately after penetration of hydrocarbon accumulations which show characteristicsconducive to a possible blow out.

    1. A careful application of these principles should provide the basis for valuable co-interpretation with thedown hole logs and should also assist in planning specific perforating schedules.

    ecause timing is extremely critical when final total depth has been reached, an in-depth interpretive analysishe instrument chart data must be prepared and available for due consideration, in conjunction with logs andther geological data, in time to influence final testing decisions.

    he paper has attempted to establish four new terms with specific meanings as they apply to the interpretation of drilud gas detection data. Therefore, remember when you are next confronted with a gas response from a gas detector,

    uestion is not "How many units was the increase?" but rather, "Was it liberated, produced, recycled or contaminatio

    FOOTNOTES

    2, 3 "The Use of Flame Ionization Detection in Oil Exploration". Mercer,

    R. F., transactions of the Second Formation Evaluation Symposium of the Canadian WellLogging Society, May 6-8, 1968, Calgary, Alberta, Canada

    4 "Evolution and Alteration of Petroleum in Western Canada", Evans, C.R., Rogers, MBailey, N.J.L., Journal of Chemical Geology, Vol. 8, 1971, PP 147-170.

    5 "Detection of Gas in Drilling Mud - Value and Limitations", Part 2, Mercer, R. F., World December, 1963.

    6 Murray, A. S., Engineering Advisor, Offshore, Imperial Oil Ltd., address to the 1973 AnnuMeeting, Canadian Oil Scouts Association.

    RICHARD F. MERCER, P.Geol.

    Mr. Mercer was granted the Bachelor of Arts degree in Geology from the Johns Hopkins University, Baltimore,Maryland. He completed one further year of graduate studies at Hopkins in Sedimentation and Sedimentary Petrogrand Petrology.

    fter serving with Shell Oil Company as a Junior Geologist on a field mapping party in Montana, he completed hisilitary requirement including graduation from the Field Artillery, Officer Basic Course conducted at Ft. Sill, Oklaho

    n 1959, he moved to Montana where he accepted a position with Continental Laboratories Inc., of Billings, a geologellsite service company. The next three years comprised field assignments at wellsites in the Northern Rocky Mountates and the Dakotas. During this period, he became familiar with all phases of geological wellsite supervision,cluding the use of gas detection equipment.

    Pgina 17 d

    19/10/tp://www.continental-labs.ab.ca/lib.htm

  • 8/12/2019 Gas Liberado

    18/18

    n 1962, Mr. Mercer was transferred to Calgary, Alberta, and was promoted to Canadian Manager. In 1967 he wasected Vice President with responsibility for Canadian Operations.

    e is a member of the Canadian Society of Petroleum Geologists, the Canadian Well Logging Society, and thessociation of Professional Engineers, Geologists and Geophysicists of Alberta.

    Pgina 18 d