Further Discussion of the 1984 Natl. Petroleum Council Studies on EOR
Authors: Earlougher R.C. 
Published: December 2007

In "Reply to Discussion of the 1984 Natl. Petroleum Council Studies on EOR," Hirasaki et al. are convinced that the OOIP values used in the NPC study are correct. If they are not correct, however, the methodology used results in a misleading, large estimate of the quantity of oil in unswept areas of the reservoirs. Their estimates of oil recovery efficiencies and oil in unswept areas are directly dependent on original oil in place (OOIP) i.e., recovery efficiency equals estimated ultimate oil recovery by conventional methods divided by OOIP. If the OOIP is high by 15%, the calculated value of oil in the "unswept area" may be overstated by 90% and the total unrecoverable oil overstated by 29%. In turn, the ultimate recovery by conventional methods would be 39% of OOIP rather than 33%.

I agree with L.F. Elkins that the OOIP value used in the NPC study is overstated and that the 33% recovery factor that has been the "party -line" number for many years is low. Hirasaki et al.'s reply would be enhanced substantially if it included a summary comparison between the former API values of OOIP and the values provided by the questionnaire returned from 18 companies for 1,300 provided by the questionnaire returned from 18 companies for 1,300 reservoirs.

In several unconsolidated and semiconsolidated sand reservoirs at depths of 2,000 ft [610 m] and below, it has been determined that the former official values for OOIP were substantially overstated because initial estimates were based on conventional core analyses with porosity determined on samples under minimal confining pressure and water saturation determined from oil-based cores. Average porosity values ranged from 32 to 38%, with water saturation values of about 18 to 30%. Subsequently, by the mid-1970's, it was determined that the in situ porosity ranged from 24 to 28%. In turn, average initial water saturation ranged from 26 to 39 %. This results in a reduction of 25% in calculated OOIP.

A report on a Shell Oil Co./U.S. DOE cost-share project, the Coalinga Demonstration Project, presented at a 1976 ERDA meeting in Tulsa by G.E. Tinker, quoted a porosity of 26%, which is substantially less than the value of 32% established in the early 1950's. The demonstration project subsequently was discontinued after polymer injection failed to mobilize oil and the OOIP was determined to be much less than originally believed. In 1978 Weinbrand reported on another U.S. DOE cost-sharing project. The in-situ porosity determined for this project was 24%, project. The in-situ porosity determined for this project was 24%, compared with the formerly believed value of 32%. In turn, the estimated ultimate recovery after waterflooding was +40% OOIP rather than 30%.

The July 31, 1989, Enhanced Recovery Week reports that Arco Oil and Gas Co. dropped a proposed pilot EOR project in South Cuyama field in California because new porosity and net pay data from a detailed study of the field indicate that the OOIP is 13 to 24% less than originally believed.

In a City of Long Beach/U.S. DOE cost-share project, the resulting porosity value was 28%. This is in close agreement with the 27% porosity value was 28%. This is in close agreement with the 27% porosity value used in the initial proposal for the porosity value used in the initial proposal for the micellar/polymer project. The long-accepted average value of porosity before 1971–72, based on conventional core analysis, was porosity before 1971–72, based on conventional core analysis, was 35%. The calculated average oil-in-place estimate was reduced from 1,900 to 1,280 STB/acre-ft [0.24 to 0.17 stock-tank m3/m3] for the HX Sand reservoir. The calculated initial water saturation, which had been based on oil-based cores, increased from 800 to 890 bbl/acre-ft [0.1 to 0.15 m3/m3].

A reduction in the estimated OOIP is not going to make any difference in the amount of oil ultimately recovered by EOR. It could, however. save the expenditure of millions of dollars by discouraging the industry from going after uneconomical quantities of remaining oil in some fields. Arco's reported action in South Cuyama is a good example.

Related papers: SPE 13239, SPE 13240, SPE 13241

Related discussions and replies:SPE 18397, SPE 20007, SPE 20009