Experimental Design Reduces Risks of a Siberian Development Having Subsurface and Financial Uncertainties

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 115575, "Experimental Design Reduces Risks of a Siberian Development With Subsurface and Financial Uncertainties," by David E. Tipping, SPE, TNK-BP; Franz X. Deimbacher, SPE, and Dmitry Kovyazin, Schlumberger; Alexey Medvedev, SPE, Landmark; and Randy Valencia, TNK-BP, prepared for the 2006 SPE Russian Oil & Gas Technical Conference and Exhibition, Moscow, 28-30 October. The paper has not been peer reviewed. Experimental-design methodologies were used to evaluate development options in a Siberian field having major uncertainties in geological, reservoir, and financial parameters. The approach demonstrated that an inverted seven-spot injection pattern would yield 11% more oil than a conventional linedrive method. For decision making, the analysis predicted that the expected net present value (NPV) of the seven-spot pattern would be 46% more than that of the linedrive pattern. Introduction The primary reservoirs in this Russian field consist of stacked fluvial, delta-distributary-channel, and mouth-bar sands. Exploration tests confirmed oil rates exceeding 120 tonnes/d. Recognizing this risk to project success, an experimental-design technique was developed for quantifying the uncertainties in oil-rate predictions. The experimental-design techniques, integrated with advanced computer technology, reduced evaluation times from months to weeks. More importantly, they provided unbiased probabilities of production profiles and NPV so that downside risks can be quantified. Development Context The proposed development area encompasses two oil-bearing zones. The shallow zone consists of sand-stones with thicknesses varying from 0.5 to 3 m with numerous interbedded siltstones. The deep zone is in an underlying incised-valley-fill complex surrounded by impermeable siltstone. Although the 2D seismic could delineate the boundaries of the incised valley, its resolution did not permit differentiating the sand bodies from the surrounding siltstone within the valley fill. A subsequent sedimentological review of core data revealed promising reservoir sands. Well tests of the valley-fill sand bodies yielded promising initial oil rates that exceeded 120 tonnes/d. However, major uncertainties existed. Although the well tests confirmed the presence of an oil/water contact (OWC), given the thin nature of the sand bodies, the OWC response on the wireline logs was suppressed by surrounding siltstones. As a result, an OWC uncertainty of 4 m existed with potential adverse effect on the estimated oil initially in place. Seismic data also revealed anomalies, possibly indicating faults acting as flow barriers. The seismic did not have sufficient resolution within the incised valley to differentiate sand bodies from the surrounding siltstones.