Abstract

The Minas Pattern Waterflood (PWF), an inverted 7 spot pattern with 23.7 acre well spacing, is in the process of being developed. The project involves drilling 417 new producing wells and converting 180 existing producing wells to injection. The project is currently 75% completed. The producers and injectors are completed in four 4 major commingled zones.

The purpose of the study is to develop an optimization strategy for PWF performance. The first step in the process is to develop reliable reservoir characterization tools to better understand the reservoir. The three tools used are lithofacies, geostatistics and 3D seismic. The lithofacies tool is used to differentiate the sand depositional environment. It is important because there is potential of a fluid flow barrier on the interfacies changing surface. The geostatistics tool is used to determine the rock properties distribution numerically. The 3D seismic tool is used to provide accurate structure maps and identify existing faults.

The second step is to design injection rates by applying pattern balancing. Geostatistical modeling is used to create porosity and saturation cubes within the pattern waterflood using open hole logs from new wells. The cubes are used to calculate moveable hydrocarbon pore volume (HCPV) within each zone. The initial injection target rates are set proportional to the HCPV to increase production from the higher oil saturation zones.

The next step is to develop surveillance and optimization recovery strategies. Several approaches are used to monitor waterflood performance. Reservoir simulation approaches are utilized to optimize the recovery. Streamtube and 3D simulations are used to study optimization scenarios. The Minas PWF production performance has been as good as expected.

Introduction

Minas field is a major oil field in Southeast Asia operated by PT CALTEX PACIFIC INDONESIA (CPI) under a PERTAMINA Production Sharing Contract. It is located in Riau Province, Central Sumatra. The field is 28 km long and 13 km wide with type of clusters are faulted anticline, covering an area of 35,000 acres with original oil in place (OOIP) of 9,023 MMSTB. The initial reservoir pressure was 930 psia and temperature 207 cF. The reservoirs depth are ranging from 1900 ft to 2500 ft. Figure 1 shows the PT CPI operations area.

Reservoir Description

Minas field is situated on the crest of the large NW-SE trending gently dipping anticline of the Central Sumatra basin. The anticline has structural dips of 1.40 to 30 on the NE flank and 20 to 40 on the SW flank. The original vertical closure and oil column are about 3,500 feet and 480 feet respectively. The anticline is associated with a North-South trending wrench fault system and its compressional component. The Minas fault, which has a right lateral movement, offsets the axis by 1 to 3 km. The Minas fault divides the field into two major portions which are the Main segment and the Northwest segment. The reservoir driving mechanism is a strong water drive.

The stratigraphic sequence in the Minas Field is entirely sandstone and shale. The oil producing reservoir in this field is a sequence of Early Miocene sandstone which constitutes the principal component of the Sihapas Group. The Sihapas Group is unconformably underlain by Pre-Tertiary Basement Rock or Paleogene Pematang Formation. The Early Miocene Telisa Formation of marine shale, overlies un-conformably the Sihapas and constitutes an excellent seal on the reservoir. The Sihapas Group is divided into 6 hydrocarbon bearing reservoirs which are the A1, A2, B1, B2, D and S sands. The waterflooded sands are the A1, A2, B1, and B2 sands.

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