Achieving good zonal isolation in wells characterized by lost circulation has been a challenge in the Wafra field of Kuwait. The field is characterized by highly fractured formations with very low fracture-pressure gradients. In the Wafra field production intervals were commonly drilled with partial or complete losses, causing risk during drilling and cementing operations and damaging the producing formation. Also, uncemented columns of casing have developed leaks as a result of corrosive formation fluids. Two-stage cementing with extended slurries and top jobs were the common practices in this field in an attempt to circulate cement to surface. This technique delays the drilling progress, and the low mechanical properties of the extended slurries do not provide the required zonal isolation.
A new cementing program was introduced to combat lost circulation in this field. A slurry technology at a density of 900 kg/m3 (7.5 lbm/galUS) was used to cement the low-fracture-gradient production zone without cement losses or formation damage. Excellent set cement mechanical properties were achieved with very low set-cement permeability. In wells where high losses were expected, a surface-treated fiber was added to the cement to bridge across the problem zone. This paper presents case histories study of several wells, laboratory designs, and field data in which these new techniques successfully treated partial to complete losses encountered during cementing operations. In all jobs using the new slurry, lost circulation problems were cured and cement was circulated to surface.
Fighting lost circulation is a costly and time-consuming process. Excessive costs can result from drilling/completion fluid and cement losses, and rig time is added for multiple stage cement jobs and possible remedial well work. Lost circulation can also have other consequences including stuck pipe, well control incidents and formation damage because of fluid loss to the production zone.
Curing these problems is critical if the operator is to control the well and drill and complete it effectively. There are numerous lost-circulation techniques in the industry, and the common objective is to contain the losses before proceeding with cementing operations. Some of the traditional treatments for these scenarios are lost circulation material (LCM) or spotting cement plugs in the open hole during the drilling phase. However, there are numerous cases in which losses are encountered just before or during cementing operations. Lost circulation can cause the primary cement job to fail, which not only increases well costs but also jeopardizes well integrity and well life.
Experiencing losses during cementing can result in poor zonal isolation caused by poor cement coverage or low top-of-cement, requiring additional casing strings and/or top-up jobs as well as subsequent workovers.
Typically, the stratigraphy for wells drilled in the Wafra field includes the shallow Kuwait series sand, which is comprised of loose sandstone from surface to a depth of about 61 m (200 ft). Deeper, the Dammam formation consists of a naturally fractured limestone interbedded with chert and dolomite (Fig. 1). The Dammam is moderately hard at the top and grows softer towards the bottom.
Between Dammam and Maestrichtian formations are the first and second Eocene formations starts from 320 m to 640 m (1,050 ft to 2,100 ft) and consist mainly of dolomites interbedded with limestone, anhydrite and gypsum. Alternated with the Eocene formations are hard and sometimes massive anhydrites with dolomite and gypsum streaks. Next, extending from about 732 m to 1,037 m (2,400 ft to 3,400 ft) are the first and second Maestrichtian formations, medium to hard fractured limestone and dolomite with shale streaks.