A Novel Stimulation Technique for Horizontal Openhole Wells in Carbonate Reservoirs: A Case Study in Kuwait

Abstract

Effective matrix acidizing in Kuwait's horizontal openhole wells is a real challenge. Reservoir heterogeneity and the length of the horizontal wells make acid placement and diversion difficult, particularly in high water-cut wells in which bottom water has broken through high permeability streaks or natural fractures. Furthermore, acid penetration is limited by the large surface area of the horizontal wellbore and is exacerbated by the relatively small injection rate imposed by the use of coiled tubing. To make matters worse, formation damage in horizontal wells is usually deep due to long exposure times. This paper discusses the application of a new viscoelastic surfactant-based, self-diverting acid system for stimulation of more than 20 horizontal openhole wells in carbonate reservoirs in Kuwait. The application also deployed a new non-particulate material that forms a highly viscous plug when it contacts water, and degrades when mixed with oil in the formation. The new fluid system is pumped prior to the acid treatment to effectively prevent the acid from stimulating high water saturation sections. The field results show significant improvement on post stimulation production owing to effective diversion and water production control, compared with wells in which a conventional polymer diversion technique was used in the past with no consideration of high water cut cases. The new technique has been selected as the standard stimulation practice in most horizontal openhole carbonate wells in Kuwait. Introduction The Wafra field, discovered by Aminoil and Pacific Western (Inter Getty Oil) in 1954, is located in the Partitioned Neutral Zone - an area between the Kingdom of Saudi Arabia and Kuwait. The mineral rights in this region are shared equally between the two countries. The field is now operated by Saudi Arabian Texaco (SAT), a 100% owned subsidiary of Chevron, and Kuwait Gulf Oil Company (KGOC). SAT operates the field on behalf of the Kingdom of Saudi Arabia under the concession agreement signed in 1949. Out of the six reservoirs in the Wafra field, the most prolific is the Lower Cretaceous Ratawi Oolite. Production from this reservoir started in 1957 with 24°API being produced primarily under solution gas drive. By 1988, the average reservoir pressure had declined to 1,780 psi from initial reservoir pressure of 3,140 psi after production of almost 600 million bbl oil. Production curtailment was being considered as an alternative to reduce the pressure decline. The field was shut-in during the Gulf War and was then brought on-line shortly thereafter with extensive development drilling program. While the field was shut-in, the average reservoir increased to 2,302 psi, signaling the presence of a moderately active aquifer. The Ratawi Oolite formation permeability varies from 1 md to 1269 md and porosity ranges from 15% to 26%, with an average of 202 md and 21% respectively. The formation temperature is 150ºF. The current reservoir pressure of the main area has dropped to 1,800 psi, whereas the east-west area remains as high as 2,200 psi due to less development.