Injectivity Decline in Water Injection Wells: An Offshore Gulf of Mexico Case Study
Authors: Sharma Mukul M. , Pang Shutong , Wennberg Kjell Erik , Morgenthaler Lee 
Published: December 2007

Injectivity Decline in Water Injection Wells: An Offshore Gulf of Mexico Case Study Mukul M. Sharma, SPE and Shutong Pang, The University of Texas at Austin, Kjell Erik Wennberg, SPE, IKU Petroleum Research, Lee Morgenthaler, SPE, Shell Oil Co.

Abstract
Injectivity decline in water injection wells can have a large impact on the economic feasibility of offshore water disposal operations. A case study is presented for an offshore Gulf of Mexico water injection project. Data are presented for five typical offshore wells for which a rapid decline in injectivity was observed due to water injection. The wells were successfully acidized every few months over a period of two years. An analysis of the data indicates that in injection wells that are not fractured, such declines in injectivity may be expected even for relatively clean injection water. A comparison of the different completion types indicates that both open hole and perforated completions would have yielded similar results. Cleaner water would have improved the situation but at a substantial cost. Fracturing the injection wells appears to be the only plausible way of substantially increasing the half life of such injectors. In cases where reservoir conditions dictate that the wells not be fractured, the economics of periodic stimulation versus the cost of installing surface facilities for cleaning up the water should be evaluated using models for injectivity decline. "What if" simulations conducted to study the impact of different process parameters such as injected particle size and concentration, injection rate and reservoir properties were found to be a useful tool in specifying water quality requirements.

Introduction
The injection of water for pressure maintenance and waterflooding applications or the disposal of produced water are becoming increasing important issues in the management of produced fluids. Most mature oil and gas producing regions of the world currently have a ratio of produced water volume to produced hydrocarbon volume in excess of 25 to 1. This indicates that large volumes of produced water need to be dealt with for disposal purposes. In offshore operations, the injection of sea water for pressure maintenance or for waterflooding is becoming increasingly common. For these reasons, it is imperative to have reliable models to predict the behavior of water injection wells.
Barkman and Davidson provided a method for estimating the half life of an injection well when the reservoir properties (kh) and water quality are specified. This method has been extensively applied to evaluate injector performance. Estimates of injector half life based on this model provide us with an order of magnitude estimate of injector half life. More recent models by Van Velzen and Van Oort provide us with a basis for using core data in predicting injector half lives. Models for injectivity decline for injection wells with various types of completions such as perforated wells, gravel packed wells and fractured wells have been provided by Pang and Sharma. These models allow us to either use core flow test data or empirically derived filtration parameters that can be used in estimating the performance of injectors. In this paper, we present a case study of the application of these models to 5 water injection wells in an offshore environment. The results of the simulation study clearly point to the usefulness of conducting such simulation studies to obtain the optimal water quality requirements which dictate the surface facilities required for maintaining specified injection rates. The case study presents a unique data set for unfractured injection wells in which the performance of the injectors has been carefully monitored over the duration of the project. The quality of the injection water and the procedures taken to treat the water have also been extensively documented. Our main focus in this paper is the injectivity decline that is caused by injected fines and their impact on injector performance.

Water Injection Project History
The goals of this water injection project were to maintain reservoir pressure in the target sand. The target sand is completely unconsolidated and only slightly compacted. Five injectors were drilled and completed below the oil - water contact in the target sand. Each well has a permeability height (kh) product in excess of 53,000 mD ft. (Table 1). P. 341^