Abstract

The Minas field in Central Sumatra is the largest field in Southeast Asia. Following completion of combined peripheral and pattern waterfloods there will be more than 4 billion barrels of oil remaining. P.T. Caltex Pacific Indonesia (CPI), Texaco, and Chevron carried out a joint effort to evaluate tertiary steamflooding of this 36 API oil reservoir. This paper describes the processes used to define potential project performance and to assess and reduce technical uncertainties.

The evaluation was carried out in two Phases. Phase 1 used existing geologic and rock/fluid properties. A high-grading process was first developed for selecting that area of the field most suitable for steamflooding. For that area, detailed geological geostatistical models and detailed numerical simulation models were constructed for forecasting production. Uncertainty ranges in key geologic and rock/fluid parameters were estimated and the sensitivity of the production forecast to those uncertainties was determined.

A Decision Analysis was carried out to quantify the economic effects of the Phase 1 uncertainties and to estimate the value of better data. That analysis justified Phase 2 which included a more comprehensive geological characterization, laboratory and field measurements of waterflood residual oil saturation, and laboratory measurements of steamflood displacement parameters. A new production forecasts based on these results had a reduced uncertainty range and suggested that substantial amounts tertiary incremental oil may be obtainable by steamflooding in Minas.

Introduction

The Minas field in Central Sumatra (Fig. 1) is operated by CPI (jointly owned by Chevron and Texaco) under a production sharing contract with Pertamina the Indonesian National Oil Company. The field was discovered in 1944 and was first produced in 1952. The reservoir is in early Miocene sandstones in the Sihapas formation at an average depth of 2,000 ft. subsea with a maximum vertical oil column of 480 ft. Average porosity is 26 %. The oil is 36 API with an average initial bubble point pressure of 235 psig. Current reservoir pressure is approximately 350 psig. Reservoir temperature is 207 F. The original oil-in-place estimate is 9 billion barrels. Minas field was initially developed on 214 acre spacing. Initial production was by aquifer drive that was augmented by peripheral water injection beginning in 1970. Starting in 1978, infilling reduced spacing to 71 Acres. In the early 1990's, phased pattern waterflood development was implemented using 71 Acre inverted seven-spot patterns. This development is approximately 70% complete and is targeted only in the thickest parts of the field. Ultimate recovery following completion is estimated to be 51% of OOIP.

The large volume of oil projected to remain after waterflooding has motivated CPI and Pertamina to consider post-waterflood recovery processes. A systematic screening process indicated that steamflooding is one process that has potential and should be studied further. This paper describes the result of that study. Focus will be on describing processes for selecting the best areas of the field for steamflooding, developing performance predictions that take into account uncertainty in reservoir and rock properties, and for focusing effort in more detailed measurements and studies.

Why a Steamflood in Minas?

Screening calculations for enhanced recovery methods in Minas show that many processes are unsuitable. For example, CO2 and hydrocarbon miscible flooding are negatively affected by the low reservoir pressure and high reservoir temperature. Chemical flooding methods (surfactants and polymers) may be technically feasible but have limited economic potential using chemicals commercially available in the U.S. because of high injectant costs. P. 257^

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