Design and Performance of a Water Disposal Well Stimulation Treatment in a High Porosity and Permeability Sand
Authors: Harding T.G. , Varner J. , Flexhaug L.A. , Bennion D.B. 
Published: March 2010

Abstract
Injectivity problems associated with produced water disposal have been ongoing in the Masila project in Yemen. Disposal wells experience an immediate low injectivity upon commencement of injection as compared to the productivity measured during pumping clean-up of the wells. It has been hypothesized that this behaviour, referred to as the check-valve effect, is caused mainly by mobile formation fines in the near-well vicinity. Injectivity often declines further because of plugging by impurities in the disposal water. Laboratory and field work have been done to test several methods of improving water disposal well performance, including the application of horizontal wells and proppant fracture stimulation of vertical wells. Another technique tried in the field was a stimulation treatment involving HCl/HF acid followed by a thin-film polymer. The intent of the treatment was to destroy potentially mobile formation fines in the near wellbore area and then to stabilize those that remained in an attempt to reduce the check-valve effect. The acid and polymer treatment was developed through laboratory core testing and was employed on a newly drilled water disposal well. The procedures and results of the laboratory work are described along with the design and implementation of the stimulation treatment. The injection performance of the well is examined relative to other disposal wells in the field. Initial results of the stimulation treatment were disappointing but the well has improved over time to become a moderately good injector. However, the results of the test have not provided enough encouragement to date to warrant further work in the field.

Introduction
The disposal of produced water in the Masila project in the Republic of Yemen has been the subject of a number of earlier papers(1–3). Field water production has been rising steadily in recent years due to the strong aquifer support in the Upper Qishn reservoir section. These reservoirs are high porosity and permeability and exhibit superior production characteristics. Current oil and water production levels are 36,566 and 198,728 m3/d, respectively. All of the produced water is disposed of through reinjection into the producing formations. Injectivity problems continue due to the nature of the formations and the quality of the disposal water. Details of the injectivity problems and investigations as to their causes have been discussed previously(1). The performance of four horizontal disposal wells and the results of four proppant fracture stimulated vertical disposal wells have been presented(2). In addition, a stud has been published of continuous high-pressure injection above formation parting pressure as an aid to injectivity(3).
Laboratory and field trials have indicated that injectivity problems are caused by a combination of native reservoir fines migration, formation plugging by injected solids and oil, and the formation of calcite scale downhole. Several authors have discussed such injectivity problems and their causes in some detail(4- 8). In addition to the use of horizontal wells and proppant fractured stimulated vertical wells, a field trial was conducted of an acid stimulation technique designed in the laboratory.