In the moderate-permeability environment of the Hassi Messaoud field of central Algeria, proppant fracturing has significantly increased production, prolonging the economic life of many wells. Unfortunately, the logistics of mobilizing a workover rig to prepare the wellbore for stimulation has extended the stimulation cycle time. Additionally, many wells have tubular problems that require intervention such as squeeze cementing or tubular replacement before the stimulation process can begin. The availability of coiled-tubing equipment in the field provides the means to stimulate the wells without a workover rig. Furthermore, when coupled with a mechanical isolation system, the coiled-tubing fracturing technique protects the production tubing from high treating pressure and abrasive proppants. In the past, the lack of a reliable isolation system has limited the success of these jobs.
Reservoir conditions throughout the field allow low-rate stimulations but induce high bottomhole pressures, which make the use of conventional packers difficult. In many environments the bottomhole treating pressure creates differential pressures across the elements of a packer in excess of 9,000 psi. Several attempts to use existing packer technology yielded unsuccessful fracture treatments, and costly fishing operations were necessary to restore the wells to production. To provide an economical solution to the problems encountered by conventional mechanical packers, a new isolation system was designed. This paper provides case histories of several wells stimulated with coiled-tubing fracturing in the extreme environment of high-pressure, deep wells in the Hassi Messaoud field and examines the engineering challenges in developing a robust isolation system.
The Hassi Messaoud field is about 800 km southeast of Algiers, in the central part of Algeria. The sandstone reservoir covers an area of 77 square miles. The producing reservoir is at 10,825 ft. The reservoir is naturally fissured with permeability from 1 to 1,000 md and has a porosity of 5 to 10%. The bottomhole temperature is 244°F.
The wells that are candidates for stimulation in the Hassi Messaoud field are not newly completed wells. Many of these older wells have problems with the tubular completion and do not have isolation between the producing tubing and the annulus. Others are cemented between the 4.5-in. production tubing and the 7-in. casing.
In the past, a well with communication problems that also showed excellent potential for fracture treatment required a separate intervention operation before it could be stimulated. A workover rig was called in to repair the leak in the tubing or casing, and then the fracture operation was completed. There are insufficient workover rigs in Algeria, and thus there is a backlog of work to be performed. If a candidate for fracture stimulation had communication in the tubular, either the well went into the queue for a workover rig, or the priority of the workover rig was adjusted to accommodate the fracture candidate. When the workover rig move to location and performed the repair or changes the completion, production was lost. The well's fracture stimulation treatment cost was then added to the costs of the intervention and the lost production. The advantage of fracturing through coiled tubing is that rig expenses are not incurred and production is not deferred.