The~aboratory methodology for determining critical gas saturation, SgC' m field depletion processes is still uncertain. Through measurements. on low-permeability rocks, we explore some of the key issues affectmg laboratory determination of Sgc. Differences between laboratory depletion and external gasdrive experiments are measured and analyzed. We also investigate the relationship between average Sgc from material balance and true Sgc, source of supersaturation, gas nucleation conditions, and dynamics of gas bubble growth in d~pletion drive experiments. We assess the use of low-bubblepoint Olls to reduce experiment time. Finally, we suggest some improvements to laboratory procedures.

We constructed a novel, state-of-the-art laboratory setup that captures several hundred pressure readings per second to study the response of cores to pressure disturbances. We used our new experimental setup to measure accurately and rapidly permeability of homogeneous cores, matrix and fractured properties of a fractured rock, and the individual segment properties of a butted core sample.

The relative permeability endpoints oflow-permeability, intermediate-wet mudstones increased with flow rate. This was because of both a decrease in capillary end effect and an increase in capillary number. Hence, conventional analysis will incorrectly describe field behavior. We recommend an improved methodology.We imaged steady-state saturation profiles at the end of different rate floods on one sample. We analyzed these profiles, which were axially nonuniform, to obtain capillary-end-effect-corrected data for each flow rate. The saturation distributions were also radially nonuniform. We characterized core-scale heterogeneity using a miscible flow test and examined the influence of heterogeneity.We conducted long-term tests on three short samples to study the higher-capillary-number regime. These tests displayed the existence of a critical capillary number. where the water relative permeability endpoint increased dramatically. The corresponding rate was over a magnitude smaller than flow rates traditionally recommended to eliminate capillary end effects.

This paper wss prepemd for Presentatlm at the 1996 SPE Permian Badn Oil and GasRecoveryCofderenceheldh Mkflend,Tertes,2S27 Mati 1998. mi5 peper wss selectedfor praswtatlon~an SW PrOgrsm~lffee folloting reviewof infomtim retained In M abstracteubmlftedby the author(e).Contentsof the peper, ae pre-ted, have notk reviewedby the Sootetyof PetroleumEngineersmd em subJedto c0m3ctiM Ly the author(s).~material, as vaanted, does nol~sserily * eny poaifffflof M SOoiefyof PetmieumEngineers., its Mmre, w~re. Paprs preeentsdd SPE meeflngaare auw to Wblbtlw Wew by Editorial-ieea of the %iety of PetroleumEngineere.Eleotr~lo qodudion, diatributi~, or dora~of any pertof thispsper for ccinmerdelpu~s wfthoulthe wTIttanoc.naenf of the Societyof PetroleumEngineersis ptilbifed. Permiaslw to mpfcduw In pffntIs restrictedto an abstrectof not mom thanw rdq illustrations msy not be copied. The ebstrad must retain msplwws aclmdedgment of Mere and by~cin tie peper was presented,Write Ubmrian,SPE, P.O. BOX SSSS2S, Richerdeon, TX 7~2S2S, U.S.A., b 01-972-952-94S5. AbstractThis study reports infectivity data for the different flow zones present during a Water-Alternating-Gas (WAG) flood. The experiments were carried out on four rock types sampled horn the Grayburg dolomite formation in West Texas. We used liquid C02-water, toluene-liquid C02-water and crude oil-liquid C02-water systems to isolate mechanisms and to address nettability concerns.We found that the infectivity of the chase water zone was comparable to the initial waterflood infectivity, and increased as the trapped C02 dissolved. The infectivity of the C02-rich zone was significantly higher than the initial waterflood infectivity. Ambient condition toluene tests and reservoir condition crude tests gave similar results for the water and C02 zones. However the crude oil, even prior to any C02 inje~ion, exhibited a strong tendency to plug the core samples. We were unable to restore permeability in the plugged up core samples by continuous flushing of different solvents.