Low Permeability Gas Reservoirs: Problems, …

1 SPE 35577 Low Permeability Gas Reservoirs: Problems, opportunities and solutions for Drilling,Completion, Stimulation and ProductionD. B. Bennion, F .B. Thomas, Bietz, Hycal Energy Research Laboratories informative purposes.~ 1~, Scx 8Iy d Petrolelm En~rs, peper - IWWII8I8d for pr~m at .. Gas T~ ~ held i1 CaIg8ry,AIIerIa. Canada, 28 AprIl - 1 May 1~.This P8f f -- ~ for p_tamn by Bn SPE Program Commfttae foIowi1g dWormadon ~ n Bn abIIract IUbmItad by 1h8 author!s). CooIIWWs d .,. peper, .~, have ~ ~ r8 IrieW8d by the Society of Petrolalm and ate con8 Cticw1 by the aulhor(s).

2 The maI8ria~ as ,.~t8d. does n~ ~arily r8ft8d any~ of 818 Society d ~ Engw-., is ~ or ~rs. PapeIW rx-II8d atSPE meetings are su~ to PIdC8tioo ,.new by EdtoriaJ Convnitl- of the Sodety ofP~um EngtI88rs. P~lslion to oopy Is ~8d to Bn absb8d d not more "- 300words. IlusIratIons may n~ '- CXIPi8d. The absb8d Shoukl oontan ~admowtedgemanl of wi\-. and by wtIom tha peper - p-'8d. Wrltellll8rl8n, SPE, 833838, RktIaIdson, TX 75083-3836, , Fax reserves of valuable natural gas and associated liquidsexist trapped in low penneability intercrystalline andmicrofractured carbonate and sandstone fonnations throughoutthe world.

3 Due to the low inherent viscosity of gas, conditionscan be such that these reserves can be recovered from theselow Permeability strata in situations where the economicrecovery of conventional liquid hydrocarbons would beimpossible. This paper describes various mechanisms whichcan influence the effective recovery of gas from lowpermeability formations and presents a variety of drilling,completion, production and remediation techniques that haveproven useful recently in optimizing the recovery of gas fromfonnations of this definition of a "low" penneability reservoir is somewhatarbitrary, but for the purposes of this paper would beconsidered to be formations which have a surface routineaverage air absolute penneability of less than 20 mD.

4 In-situreservoir condition permeabilities in these types of reservoirsare generally less than 1 mD and can range down into themicroDarcy range (10-6 D) in many the emphasis in this paper is specifically on lowpermeability gas reservoirs, much of the infonnation presentedis also applicable to higher Permeability gas is the Challenge?If we consider what could cause uneconomic production ratesfrom a low Permeability gas bearing formation, the optionsgenerally will fall into six categories, these being;1. Poor reservoir quality - period!2. Adverse initial saturation conditions3. Damage induced during drilling and completion4.

5 Damage induced during hydraulic or acid fracturing5. Damage induced during kill or workover treatments6. Damage induced during production operationsAbstractAs the industry seeks to increasingly exploit reserves of naturalgas contained in low Permeability intercrystalline sandstoneand carbonate formations 20 mD in Permeability ) manyquestions have arisen as to the optimum practices to drill andcomplete horizontal and vertical wells in these systems as wellas the best techniques to hydraulic or acid fracture theseformations to obtain economic production paper provides a summary of recent work which hasbeen conducted in the diagnosis and remediation of problemsassociated with tight gas reservoirs.

6 Information on theimportance of reservoir quality assessment and initial saturationdetermination is presented as well as a detailed discussion ofcommon damage mechanisms which can affect the productivityof tight gas formations. These include fluid retentionproblems, adverse rock-fluid and fluid-fluid interactions,counter-current imbibition effects during underbalanceddrilling, glazing and mashing, condensate dropout andentrainment from rich gases, fmes mobilization and solidsprecipitation. The impact of these problems during drilling,completion, workover and kill operations is reviewed andsuggestions presented for the prevention and potentialremediation of these examples of where these problems have beenobserved in 23 different common Western Canadian lowerpermeability gas horizons are presented in a summary formatSPE 35577 LOW Permeability GAS RESERVOIRS: PROBLEMS, OPPORTUNInES & SOLU110 NSFOR DRILLING, COMPLE110N, STlMULA TlON AND PRODUcnON1 Each of these categories will now be reviewed in more Poor reservoir Quality - Period!

7 The saying "You cannotmake a silk purse out of a sows ear" has definite applicabilityto the field of tight gas exploitation. Through the use ofappropriate drilling, completion and in some cases large scalefracturing techniques, some operators have been amazinglysuccessful in obtaining economic production rates fromformations exhibiting in-situ matrix permeabilities of as low as10-6 D. Some reservoir applications, however, are doomed tofailure by the simple virtue of the fact that, no matter howsuccessful and low damaging our drilling and completionoperations, sufficient in-situ Permeability , pressure, reserves orall of the above are not present to obtain economically viablewells.

8 The prime objective of this paper is to identifycandidates of this type, in comparison to those where thejudicious application of the appropriate technology can obtaineconomic wells. In general, no documented evidence exists ofeconomic production from formations having aninterconnective matrix Permeability of less than 10-6 D (evenin the presence of successful large scale fracturing treatments).If an extensive micro or macrofracture system exists and thewell can be completed without impairing the conductivity ofthe natural fractures, then documented cases exist where viableproduction has been obtained from source matrix of lowerpermeability values than 10-6 D.

9 Penetration of a lessextensive microfracture system by a conventional hydraulic oracid fracturing treatment, to augment inflow, has also beensuccessful in some sub microDarcy reservoir successes in these situations have been highly dependenton geological control and being able to effectively predict thelocation of and penetrate the pre-existing natural fractures withthe wells or fracture treatments. Where such penetration hasnot occurred, the wells have generally been non Accurate ~ data is often unavailable for these fonnations as,due to high capillary trapping effects, many of these zones donot produce mobile free water to facilitate accuratecompositional and ~ Water saturations evaluated from cores drilled with waterbased fluids are often elevated due to core flushing andspontaneous imbibition effects.

10 Water saturations obtainedusing air or nitrogen as coring fluids are often lower then truein-situ values due to localized heating during the coringprocess and core desiccation. Oil-based coring fluids can resultin good water saturation detenninations, but flushing can affectthe accuracy of the measurement of the magnitude of anytrapped initial liquid hydrocarbon saturation which may bepresent in the variety of techniques are available to measure in-situ waterand oil saturations, the best and most reliable being specialitycoring programs in the producing zone of interest. In zonescontaining only an initial water saturation, radioactively traced(deuterium or tritium) coring fluids can provide an accurateevaluation of initial water saturation when coupled with lowinvasion coring technology (Ref.)