For the past several years, P.T. STANVAC INDONESIA has been actively engaged in an infill drilling program in its old field onshore Sumatra. The primary purpose of this program is to identify and recover remaining accumulations of oil which can not or will not be drained by existing wells.
This paper reviews the results of such infill drilling in Stanvac's Raja and Abab fields in South Sumatra. The program has already provided a sizeable increase in oil production and reserves in both fields while extending the productive limits well beyond what had been considered to be the field limit.
The program has shown that original field development drilling on 80-acre spacing did not define all of the hydrocarbon bearing zones nor establish all of the drainage points required to efficiently recover the moveable hydrocarbons in the stratigraphically complex Talang Akar formation. Improved economics resulting from higher energy prices has provided the incentive required to pursue the development of these incremental reserves.
P.T. STANVAC INDONESIA or "Stanvac" is a company whose P.T. STANVAC INDONESIA or "Stanvac" is a company whose shares are equally owned by Exxon and Mobil OH. The company is operated by Exxon and conducts its activities in the exploration for and production of oil and gas onshore on the island of Sumatra. There are two general areas of operations. In the south, near the city of Palembang, Stanvac operates an area covering almost two million acres under the terms of a Contract of Work. This area has produced over 640 million barrels of oil since production was produced over 640 million barrels of oil since production was started in 1926. In the central portion of Sumatra, Stanvac operates another two million acres under Contract of Work terms and this area has produced over 220 million barrels of oil since initial production in 1957. Stanvac is also operator of areas in both south production in 1957. Stanvac is also operator of areas in both south and central Sumatra under Production Sharing Contract terms, where it is actively engaged in exploration activities (Figure 1).
Total production from the Contract of Work areas presently averages 36 thousand barrels of oil per day from 480 wells in 28 fields. Water-floods in four of the largest south Sumatra oilfields and pressure maintenance by water injection in the larger central Sumatra fields are in progress. In addition, Stanvac also delivers about 100 million standard cubic feet of natural gas from the South Sumatra area to Pertamina and to the Pusri fertilizer plants in Palembang Under various gas supply agreements. Palembang Under various gas supply agreements. The drilling of development wells has been and continues to be one of Stanvac's major and most profitable activities. For the past five years, the company has spent about 76 percent of its annual development budget on the drilling of new wells and the construction of associated producing facilities. In return, it derives about 14 percent of its annual oil production from the new wells drilled during the year. The profitability of the development drilling program is highlighted by very rapid payout times and high rates of return. A typical well can be expected to generate about 200 thousand barrels of additional oil reserves upon completion, but requires only a fraction of that amount to pay out drilling and completion cost. The following pages review the results of such drilling in two Stanvac's oldest oilfields.
Low drilling cost and high prices combine to make this type of endeavor most attractive. The cost of drilling and completing a new well by Stanvac averaged $38 per foot in 1977 and increased to $55 per foot in 1980. The main reason for these low drilling cost is that per foot in 1980. The main reason for these low drilling cost is that in accomplishing its annual drilling program, Stanvac uses three of its own rigs on a full time basis and only where necessary makes use of a contract rig to complement the three company rigs. High crude oil prices during the late seventies further improved the economic incentive for drilling development wells to the point where the average 1980 well would pay out after producing 20 thousand barrels of oil. This means that a new well producing 100 barrels oil per day would pay out its drilling cost after just seven months. Since per day would pay out its drilling cost after just seven months. Since infill drilling programs normally have their share of dry holes, an "average" well would require a slightly longer period to pay out, depending on the success ratio of the program.
Prompted by the attractive economics, Stanvac has intensified the Prompted by the attractive economics, Stanvac has intensified the search for infill drilling opportunities in its old. mature oilfields and at present allocates about 50 percent of its drilling budget to this effort. The rationale for drilling in mature oilfields is usually to improve drainage of existing reserves or to accelerate the recovery of such reserves. The former is applied when it becomes apparent that existing drainage points will be unable to recover all of the remaining reserves, while the later is done in the expectation of improving the present value of remaining reserves without necessarily changing over-all recovery. Furthermore, where hydrocarbon accumulations are highly stratigraphic in nature and existing well spacing increases the likelihood of finding new sand lenses not encountered by present wells, the incentive for infill drilling becomes especially attractive because of the potential reserve additions involved. It is this prospect which has been the main reason for initiating an active infill drilling program in the Raja field in 1976.
The stratigraphic nature of hydrocarbon entrapment in the Raja and Abab areas has been well documented by analysis of cores and correlations between wells. The cores indicate a series of depositional environments in which potential stratigraphic traps were created when Talang Akar sandstones were deposited in the form of meandering-stream point bars (in a generally north-south direction) and coastal beach deposits (in a generally east-west direction). The well-to-well con-elations further demonstrate the onlapping nature of Talang Akar deposits onto the pre-Tertiary basement surface in a progressively northward direction. Stratigraphic traps were formed as Talang Akar sandstones pinched out up-dip against basement. The relief of pre-Tertiary pinched out up-dip against basement. The relief of pre-Tertiary basement at the time of Talang Akar deposition therefore is highly significant, as it controls to a large extend both the undip stratigraphic pinchout of fluvial sandstones as well as the trend of coastal beach deposits, both of which form important hydrocarbon traps in the Raja and Abab areas (Figure 2).
Stratigraphic type reservoirs are predominant in the old Raja and Abab fields in south Sumatra and, as following discussions show, it is in these two mature oilfields that infill drilling activities during the past five years have generated new production peaks and significant past five years have generated new production peaks and significant reserve additions.