Use of a Well Model To Determine Permeability Layering From Selective Well Tests

Abstract

This paper describes the use of a numerical well model to determine permeability layering from selective drillstem and production tests for two wells in two Middle East reservoirs. Permeabilities of various geologic layers of a stratified reservoir may be determined by careful planning of selective drillstem tests coupled with a well model to interpret the pressure behavior of each test. Introduction The Middle East Reservoir A is a complex carbonate reservoir exhibiting considerable lateral and vertical lithological changes with consequent variation in reservoir characteristics. Based on three previous wells, six geologic units were identified, only five of which are considered productive. Drilling of Well 4 was planned to include drillstem tests of each of these five geologic units to determine their flow characteristics. After the well was completed, it was perforated and acidized and long-term production tests were performed. A numerical well model was developed for use in interpreting these tests by matching pressure buildup performance. This numerical model incorporated all the known geological zonation based on geologic analysis of the four wells drilled to that time. A three-dimensional well simulator was used in this modeling. However, it was used in a two-dimensional radial mode.Later this same technique was applied to an infill well in Middle East Reservoir B. This reservoir has been producing for several years. It is a carbonate reservoir and has considerable variation in lithological characteristics in both the lateral and vertical direction. Selective drillstem tests were conducted on Well 149 at the time it was drilled. Well 4 Drillstem Tests A schematic of the drillstem test procedure is shown in Fig. 1. After each geologic unit was drilled, a packer was set isolating that unit while a drillstem test was conducted. The well was produced for time periods ranging from 4.4 to 7 hours with two pressure recorders opposite the producing intervals. The well then was closed, and the buildup pressures were recorded.Reservoir A is highly undersaturated such that single-phase oil flow occurred at all times in all tests. While the reservoir has an aquifer, the well is located on the crest of the reservoir a considerable distance away from the water zone. Cores were obtained from all geologic units for Well 4.A radial well model was constructed to simulate the performance of this well. To have the first grid point as close to the wellbore as possible, it was selected 1 ft (0.30 m) from the wellbore. Six additional radial segments increase logarithmically in size to encompass a total drainage radius of 4,000 ft (1219.2 m). This radius is roughly comparable to the distance from the well to the leading edge of the oil/water contact. A constant-pressure outer boundary was assumed. However, for the short-term drillstem tests, it did not matter whether the outer boundary was sealed or at constant pressure due to the short duration of the tests. JPT P. 2023＾