Prediction of Specific Productivity Index for Sihapas Formation in Uncored Wells of Minas Field Using Limited Available Core and Log Data
Authors: Rinaldi . , Djauhari Harris H. 
Published: December 2007

Determination of how much production contribution from individual zones in amulti-layer environment is very critical in optimizing initial commingled completions and subsequent workovers. The conventional Individual Zone Production Testing (IZT) method to determine the maximum well productivity is very inefficient due to unnecessary repetitions in workovers resulting in higher operation cost.

The possible maximum production rate of each layer can be estimated by calculating its productivity index and applying the maximum allowable pressure drawdown. Data to calculate the productivity index for each layer is not always available. Production logging (PLT) or IZT, the source of data is not run in every well. These common problems could be addressed by instituting a production zone allocation system.

This paper reports a practical approach to estimate absolute horizontal permeability and the specific productivity index (SPI) by correlating to density porosity (d) and Volume Shale of Gamma Ray Index (VshGR) for Sihapas formation in Minas Field. The d and VshGR are calculated from open hole log data, which is usually available. The correlation is principally generated by using the permeability - density porosity transform, relative permeability curve and Darcy's law for radial flow in porous media. The calculated SPI is a fairly good match with actual field data obtained from PLT and IZT. In some conditions the calculated SPI is under estimate compared to actual data. Thisslight disagreement will also be reviewed in this paper. The method to generate this practical correlation can also be applied to data from other fields.

Introduction

Estimation of formation fluid or oil production from each layer is a fundamental common problem in the multi-layer oil production system, such as Minas Field. Inter layer communication behind cement, and water cross-flow through channels in the cement between casing and formation can even make it worst. Production allocation system can be expected to ease the problem in the reservoir management. However, it is very complicated to generate the system due to reservoir heterogeneity and complexities with very limited data available.

Minas is a major oil field in Indonesia located about 33 KM northern Pekanbaru, the capital city of Riau Province. This field, operated by PT. Caltex Pacific Indonesia with a Production Sharing Contract (PSC) system, is agently dipping anticline which is 28 km in length and 7 to 13 km in width. Original oil-in-place is estimated to be 9.0 billion STB.

Minas field was discovered in 1944, and the first oil well was initially completed and put on production in 1952. The initial field development was 214acres spacing. To enhance the natural aquifer pressure support, peripheral water injection was started in 1970. A closer well spacing of 71 acres field development was begun in 1978. In order to increase oil recovery, 23.7 acrespacing of Phase 1 inverted seven spot waterflood has been implemented in high graded areas of the field since late 1993. The ultimate oil recovery of the waterflood project is expected to be 51 % of the original oil-in-place.

The average recent field production is around 250,000 BOPD and 94.9 % watercut from commingling of several layers of Sihapas formation. The Minas fluid production is artificially lifted by Electric Submersible Pumps (ESP) from 679active wells, that will increase significantly in the near future when the planned tertiary oil recovery processes with a closer well spacing is implemented.

Sihapas formation of early Miocene age is overlain by the Telisa formation of the Middle Miocene age, and underlain by Pematang formation of the Paleoceneage. The Reservoir sands of Sihapas are divided into six major units referred to as A1, A2, B1, B2, D and S sands separated by five major shales.

P. 183^