Zonal isolation in high-loss wells can be extremely problematic. A new lightweight fibered cement system helps solve severe lost circulation problems.

An extended cement slurry system used in conjunction with specialized fibers has been applied successfully to solve severe lost circulation problems for Joint Operations Company (Kuwait Oil Company/Saudi Arabian Texaco - JOC) in Wafra field, Kuwait. Cement slurries having densities as low as 7.5 lb/gal were achieved while eliminating the need for foam and retaining the superior properties of the set cement. On Wafra wells experiencing severe to complete fluid losses during drilling and cementing, the 7.5 lb/gal slurry and fiber combination was pumped for the first time anywhere and achieved zonal isolation across the Dammam and first Maestrichtian formations with a single stage cementing operation.

Lost circulation

Fighting lost circulation is a costly and time-consuming process. Excessive costs 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 also can have other consequences including stuck pipe, well control incidents and formation damage because of fluid losses to the producing 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 circulating a high concentration of 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.

Wafra field, Kuwait

Typically, the shallow 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 200 ft (61 m). Deeper, the Dammam formation consists of a naturally fractured limestone interbedded with chert and dolomite (Figure 1). The Dammam is moderately hard at the top and grows softer toward the bottom. Between the Dammam and Maestrichtian formations are the first and second Eocene formations (1,050 ft to 2,100 ft or 320 m to 640 m) consisting 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 2,400 ft to 3,400 ft (732 m to 1,037 m) are the first and second Maestrichtian formations, medium-to-hard fractured limestone and dolomite with shale streaks.

Wafra's shallow lost circulation problems occur in the Dammam formation through the Maestrichtian formations. Based on experience from other Middle East operations, this shallow section of the hole is drilled blind with water. However, using water as the drilling fluid requires viscous sweeps and LCM pills to maintain adequate hole cleaning. The higher equivalent circulating densities (ECD) imposed by the viscous fluids induce additional losses, thus further exacerbating lost circulation problems.

The main classification of the lost-circulation zones at Wafra are natural and induced fractures (Figure 2). Horizontal or vertical, natural fractures normally have little or no width until they are widened by applied hydraulic pressure. A natural fracture in hard consolidated formations must have a finite supported width to accept fluid. Therefore, fluid losses to the natural fracture are slight until the required hydraulic pressure has been applied to open it and allow fluid into it. This phenomenon would be indicated by a sudden loss of returns.

The problem encountered

Heavy losses of circulation in the Dammam are expected at depths from 200 ft to 800 ft (61 m to 244 m). As stated previously, this formation is drilled blind using water as the drilling fluid, with well control maintained by pumping water through the annulus. Some losses also can be expected in the Eocene formations, especially when they are depleted. Because of the shallow depths (less than 4,000 ft or 1,220 m) , the losses in the hard, consolidated Maestrichtian dolomitic limestone formations are believed to be attributed to horizontal natural fractures that propagate and get wider with applied hydrostatic pressure of viscous fluids that seep into the depleted, low pore-pressure reservoir. Also, induced fractures can occur in the Maestrichtian and below provided the borehole pressure exceeds the formation parting pressure. The difference between a natural and induced fracture is that the induced fracture requires an initiation pressure to open it. Once opened, there is essentially no difference between induced and natural fractures from the viewpoint of treating lost-circulation problems. When drilling, losses are indicated by a gradually lowering of mud in the pits. Losses can become severe as drilling is continued and more fractures are exposed. The point often is reached when a complete loss of returns is experienced.
In the past, if a Wafra well experienced partial losses, the practice of placing a viscous pill at bottom and allowing it to soak for a time period provided good results because filtration from the drilling fluid deposited solids in the fractures. However, this practice had little to no effect on complete losses. Numerous attempts to prevent severe losses as they occurred involved significant investments in both time and money yet had limited success. These efforts included LCM and viscous pills whose added costs negatively impacted well economics while not effectively containing the losses.

Consequently, after drilling, Wafra operators were faced with the subsequent challenges of cementing under total loss conditions. Multiple stage cementing and extended slurries were attempted as solutions for severe lost circulation problems. However, after the cementing operation, several top-up jobs using about 100 to 150 bbl of slurry were required to bring cement to surface. Weak points developed in the casing sheath around the stage collar. Failure is always a potential problem in this area. Shallow water zones cause corrosion in unprotected or inadequately covered sections of the cemented casing.

Consequently, the need was created for a new type of treatment. Schlumberger Ultra LiteCRETE systems mixed at densities just above that of water had been used widely for wells experiencing severe losses in Kuwaiti fields, including Wafra. Here, the second Eocene and Maestrichtian formations had been cemented successfully for some time using the near-water density slurry systems. Based on these successful well applications, as measured by circulating cement to surface in a single stage, an analysis of the Dammam and first Maestrichtian losses at Wafra concluded that an ultra low density cement slurry having optimized particle-size distribution could be used to solve lost circulation problems by sealing the fractures and thus preventing fluid from entering them and extending them further.

The solution applied

The first of Wafra's ultra light cement wells was drilled in February, 2003, using JOC's Rig KDC-17. The well was experiencing 100% fluid losses during drilling the shallow section across the Dammam and first Maestrichtian formations. After unsuccessfully attempting to cure the lost circulation problems with viscous and LCM pills, it was decided to proceed with the cement job. To increase the chances of achieving good zonal isolation and bringing cement to surface, the operators teamed with Schlumberger to modify plans and develop a program using Ultra LiteCRETE cement and CemNET fibers at an unprecedented low density of 7.5 lb/gal. The slurry and fiber combination was pumped with full circulation gained during displacement as evidenced by the 10 bbl of cement system that was circulated to surface.

Combining the two technologies of the ultra low density cement slurry and specialized fibers resulted in an ultra lightweight fibered cement system having unique attributes tailored for solving the severe or total lost circulation problems faced by JOC. The cement slurry can be efficiently mixed and pumped at or below the density of water and, once set, provides compressive strength and permeability properties superior to other lightweight systems or cement alternatives such as foam. What enables the advanced cement to have ultra low-slurry densities while retaining production-quality properties are sized lightweight particles distributed in a manner that maximizes solids content and packing volume fraction (Figure 3). The graded spherical grains reduce not only the slurry's density, but also the cement column's hydrostatic head in the well. This is a particularly important quality at Wafra given that its formations are largely depleted through years of production, resulting in lower reservoir pressures.

The specialized fibers used are comprised of an inert fibrous material that does not alter cement properties. The fibers can be added to slurries in the mixing tub, thus enabling their addition to only the slurry portion targeted for zones where losses are anticipated. Once dispersed in the slurry and placed across thief zones, the fibers create a physical network that bridges them off so cement circulation resumes during treatment (Figure 2). Further, the slurry and fiber technologies work in synergy, with the fibers effectively forming a mat at the point of lost circulation and the sized particles of the cement slurry quickly bridging the gap between the pore spaces and voids of the fibers.

To date, the Wafra wells cemented using the ultra light cement system have permitted complete cement coverage to surface with little or no losses. As such, its use has enabled effective zonal isolation with a single stage cementing operation, thus reducing the time, costs and potential problems related to cementing that normally takes longer on a two- or three-stage operation. This has reduced operating time and costs on Wafra wells by saving 8-10 hours per well, respectively.

The lower densities allowing the ultra light cement to be readily circulated back to surface have resulted in an almost 90% reduction in top-up jobs that were needed previously to provide adequate support for the casing. Reducing excess cement slurry pumped in anticipation of losses also has optimized cement volumes. Moreover, the success of the 7.5 lb/gal slurry system in achieving and maintaining casing integrity has eliminated the need for costly stage collars/tools as well as the subsequent costs associated with workovers and remedial cementing required to repair any leaking stage tools and corroded casing. More durable zonal isolation also was achieved through the cement system's improved resistance to chemicals and corrosive fluids.

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