Summary
The main focus of this paper is the applications of polymer treatment in controlling injectivity and the improvement of the near-wellbore injection profiles.
Reservoir high-permeability zones causing distortions of water injection profiles at the wellbore, and the possible existence of layer barriers, were identified with conventional investigation tools such as openhole logs, production logging tools, formation, and microscanners.
A polymer screening procedure was put in place so that appropriate polymer formulations that can reduce fluid mobility in the high-permeability zones at the near-wellbore region can be identified. The use of these polymer formulations facilitated the improvement of wellbore injectivity profiles.
The field performance of the polymer treatment program was ascertained by the use of a monitoring program that combined the application of the production logging tools and well-test analyses.
Introduction
The presence of high-permeability streaks and fractures are common occurrences in stratified carbonate reservoirs. If these reservoirs are subjected to patterned waterflooding operations, it is likely that the highly permeable zones will accept most, if not all, of the water injected. This tends to distort the injectivity profile at the wellbore, and raises the possibility of an eventual poor reservoir coverage and sweep efficiency. Therefore, a successful waterflood project necessarily demands a clear understanding of reservoir characteristics with a view to identify occurrences of high-permeability streaks, channels, and layer barriers, so that adequate remedial actions that can reduce the adverse effects of these reservoir characteristics on waterflood performance can be put in place. One type of remedial actions that has recently gained prominence is the use of polymer1–3 and nonpolymer4,5 agents for injection/production wellbore profile modification for vertical wells. A case history for horizontal well application of polymer treatment was recently documented in the petroleum literature.6
This paper presents the screening and evaluation procedures for the selection of a crosslinked polymer formulation for use in profile modification in water injection wells. The post-polymer injection monitoring of the polymer-treated injection wells and the surrounding producers also are presented.
Background History
The reservoir under consideration is a mature reservoir under five-spot pattern water injection. The injection pattern size has been under continual modification by implementing a carefully planned program of infill drilling and the conversion of some original producers to injectors. This progressive review of pattern size has reduced the distance of producers to injectors from an original value of 2.8 km to a current value of about 1.4 km. Simulation studies indicated that water production from this reservoir is not expected to start before 2010. However, it has been observed that some of the producers have started experiencing premature water breakthrough. Analysis of produced water confirmed that water being produced by these wells comes largely from injected seawater.
Field-wide investigation as to the cause(s) of premature water breakthrough in some wells was initiated. These investigations involved a complete re-evaluation of all available open- and cased-hole logs of all producers and the neighboring injectors within and around the pattern in which premature water breakthrough has been observed. Special considerations were given to production logging tool (PLT) logs, particularly newly acquired PLT, pulse neutron logs, and core data from infill drilling and dynamic testing programs such as MDT, RFT, and transient well tests. This data acquisition program was designed to correlate injection profiles of the various water injection wells in the reservoir.
The investigation confirmed that the earlier static geological model that splits the reservoir into six layers (A, B, C, D, E, and F), separated by five stylolitic intervals (S1, S2, S3, S4, and S5, respectively, as shown in Fig. 1) may require further refinements. Furthermore, observations from core and RFT data showed Layers A and B to be characterized by zones of high-permeability streaks. Evaluation of water-cut profiles throughout the entire reservoir did not seem to support any directional fluid drift caused by reservoir-wide directional permeability. PLT results did, however, confirm that most of the water injected into most of the injectors do preferentially and disproportionately go into the top reservoir Layers A and B. The lower reservoir Layers C, D, E and F show no evidence of any quantifiable injection water intake.
PLT surveys of the producers also show that Layers A and B might have accounted for all the water production at the producers. This could suggest that the high-permeability streaks are controlling the waterflood flow path inside the reservoir.
In most practical cases of waterflood, the ideal piston-like displacement by water could hardly be achieved. This is more so for cases where high-permeability streaks, thief zones, or high-conductivity fractures cause a disproportionate amount of recoverable oil reserves that can be bypassed, resulting in poor volumetric sweep efficiency. In order to optimize the water injection program by reducing the incidence of premature water breakthrough, vertical conformance of water intake profiles at the injectors must be improved. Therefore, a reduction in the water injection into the upper high-permeability Layers A and B of this reservoir while increasing intake into the less-permeable lower Layers C, D, E, and F could improve the water injection performance of the reservoir substantially.
In view of the above, it was decided to implement a polymer treatment program on some selected injectors, particularly the original high-capacity injectors where premature water breakthrough has been observed within its flood pattern. In doing this, it is hoped that injected polymer will infiltrate the high-permeability streaks in Layers A and B and reduce the injectivities of these layers by selective permeability reduction. If subsequent injection wellhead pressure remains the same, it is expected that the water intake into Layers C, D, E, and F will proportionately increase.
