Evaluation of Calibration Treatments, for Frac Pack Completions, in Offshore West Africa
Authors: Gulrajani Sunil N. , Tibbles Raymond J. , Nolte Kenneth G. 
Published: December 2007

Frac Pack treatments were selected as the most effective completion for the combined benefits of sand control and stimulation from very high permeability, oil-bearing, offshore formations in West Africa. Extensive formation evaluation tests, consisting of step rate and step-down tests, calibration treatments and laboratory core data analysis, were performed prior to the early frac-pack completions. Treatment evaluation, however, was complicated by the inability to correctly characterize calibration tests for estimation of fluid leakoff and the corresponding fracture efficiency values. The presence of near-wellbore effects further compounded the difficulties in analyzing calibration treatment pressures. On most of these applications the propped treatment screened out prematurely and thus had to be terminated early.

Previous mini-fracturing pressures were re-analyzed for a consistent evaluation of the calibration tests. A comprehensive pressure analysis methodology was developed for fracture evaluation. Post-fracture closure behavior was examined in addition to traditional analyses of pressures measured during treatment injection and shut-in. An inversion algorithm was also used to objectively predict formation properties and fluid parameters by matching mini-fracturing pressures. These results, log interpretations and laboratory core data were then integrated to improve the predicted fracture parameters and hence provide reasons for treatment failure. This approach investigates all stages of fracturing behavior, thereby providing formation mechanical (stresses, modulus), fluid efficiency (leakoff coefficient) and production (permeability) parameters governing optimal fracture design.

Comprehensive case studies, describing this evaluation methodology for multiple injections in different formations with varying mechanical and fracturing fluid properties, are presented. Calibration treatments are first analyzed. Propped treatment pressures are next predicted and compared with field measurements. These evaluations are then applied to properly design and execute subsequent frac-pack completions, which improved the success of such completions.

Introduction

Frac Pack stimulation treatments were introduced in West Africa to provide a combined well-stimulation and sand control completion. This technique was preferred over a conventional gravel pack treatment because of a lower completion pressure drop and resulting skin, and thus a higher productivity. The Frac Pack technique combines a short highly conductive fracture treatment with a gravel pack. Creation of this fracture reduces the completion pressure drop by:

- Providing a highly conductive flow path to the wellbore, which bypasses any near wellbore damage.

- Changing the flow path into the well, which will minimize any chance of perforation or gravel pack plugging due to fines or formation sand migration.

- Maximizing the quantity of sand placed in each perforation. This insures that all of the perforations are packed with high permeability gravel instead of formation sand.

During the introductory phase of this technique, it was quickly realized that calibration treatment pressure analysis posed particular problems in these high permeability wells. Fluid efficiencies were much lower than routinely experienced for treatments or even Frac Packs performed in other parts of the world.