Abstract

Combining multiple tools or processes can provide creative answers to old problems. An Algerian well was producing from two perforated intervals. The upper zone was previously fractured; the lower zone was underperforming. Stimulation treatment using a hydrofluoric-based acid system was designed for the lower zone to significantly increase the well production. Although there was confidence in the stimulation design, certain operational challenges needed to be addressed. To maximize the economics, the workover operation had to be rigless. The decision was made to use coiled tubing with a newly designed through-tubing inflatable packer to mechanically isolate the zone and deliver the harsh fluids required for the stimulation. The next challenge was setting the packer in a 3-m window between the two zones. If the packer were not correctly positioned the treatment, undeniably, would be misplaced. A wireless casing collar locator (CCL) with pump-through capability was chosen to provide accurate, real-time depth readings prior to setting the inflatable packer in a modular combined tool string.

The designed workover required only a single trip in the well, and the production tubing did not have to be retrieved. With the creation of a real-time correlation log, the inflatable packer was set on depth, and the hydrofluoric-based acid system was pumped into the underperforming zone. Post treatment, the oil production more than tripled. Two technologies were combined in an innovative application that met all operation objectives.

Introduction

The Hassi-Messaoud field, extending over 2,500 km2, is developed in a flattened anticlinal structure located on the northern part of the El-Biod-Hassi-Messaoud elevation. The production is associated with a thick sandstone section, located at a depth of 3,300 m, consisting of four distinct formations and a transition zone. Overall, the field is divided into 25 producing zones that differ in geological and petrophysical properties. The reservoir pressure drive in the field uses water and gas injection.

Completion types in the field include openhole, slotted liner, mixed liner, cemented liner, and perforated liner, with or without concentric pipes. Most of the original openhole and slotted liner completions have been worked over to change to cemented and perforated liners. During most workover, re-completion, and snubbing operations in the Hassi-Messaoud field, the well is killed for operational and safety regulations. The well killing most often uses oil base mud or workover fluids. These fluids are often incompatible with formation fluids and can cause severe damage to the near-wellbore matrix.

Matrix acidizing treatments were introduced into the Hassi-Messaoud field in the early 1980s to restore and optimize productivity after workover or snubbing operations. Currently approximately 50 matrix acidizing treatments are performed each year.

The well MD 264, located is zone 23, was originally drilled in the late 1970s to a total depth of 3,503 m. The openhole completion produced at a rate of 13.7 m3/h. A workover operation, completed in the mid-1990s, ran a 4 ½-in. cemented and perforated liner in the well. Soon after completion, a conventional fracturing operation performed on the first set of perforations (depth interval 3,406 to 3,418 m) did not improve production. Two additional sets of perforations were added a few months later at depth intervals 3,453 to 3,464 m, and 3,421 to 3,451 m, with a 3-m gap at 3,430 to 3,433 m (see Fig. 1.) Still no production was achieved until a matrix acidizing operation was performed, resulting in an oil production rate of 2.37 m3/h.

A nodal analysis based on a buildup test performed in 2001 indicated a skin damage value of + 3 with a simulated potential to improve production to 3.90 m3/h.

The top layer had already been fractured without an improvement in production; therefore, it was decided to use a through-tubing inflatable packer for isolation to selectively stimulate the well by pumping a treatment of hydrofluoric based acid only in the lower interval (3,433 to 3,464 m).

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