Acid Fracturing and Hydraulic Fracturing Applied in a Single Well for a Deep Carbonate Reservoir

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 226647, “Acid Fracturing and Hydraulic Fracturing Applied in a Single Well for a Deep Carbonate Reservoir Appraisal: The Completion, Perforation, Stimulation, and Data-Gathering Experiences for Seven Zones, ” by Mathieu M. Molenaar, Naveen K. Singhal, and Mark E. Brady, Shell, et al. The paper has not been peer-reviewed. _ Acid fracturing and hydraulic fracturing were used to stimulate and appraise a challenging deep, low-porosity carbonate reservoir with a complex combination of matrix background fractures and some large fault corridors and fault-associated fractures. The execution of the stimulations proved challenging because of the high strike/slip stress regime. Data from the prestimulation diagnostic tests was used to successfully plan and optimize each stimulation. Learnings and experiences for designing and execution of proppant fracturing, as an alternative to matrix or fracture acidizing in these complex carbonate formations, are presented. Introduction To date, five wells have been drilled in the appraisal field. Four of the wells have reached the Eocene to Cretaceous fractured carbonate reservoir. The gas-condensate reservoir fluid contains hydrogen sulfide (0. 3–6. 7 mol%) and CO2 (0. 4–2. 1 mol%) and produced at gas/oil ratios of 2, 400–3, 700 scf/stb. The wells have penetrated all structural domains. Short-term well tests have been conducted in Wells 1 and 2, and short-term and extended well tests were performed on Well 4. Well 5 penetrated the forelimb of the structure and is the deepest of the five exploration and appraisal wells drilled. Well 5: Drilling Outcomes, Objectives, and Stimulation Targets Well 5 was drilled vertically through the overburden to the top of the reservoir without encountering major losses. The long reservoir section approximately 800 m true vertical depth (TVD) was drilled deviated (45°) to a total depth of 5, 623 m TVD subsea. The well azimuth in the reservoir was in a southeastern orientation with the aim of intersecting major fracture corridors in the reservoir. While drilling the reservoir section, no losses were encountered. When Well 5 was drilled, key subsurface uncertainties existed regarding the recovery per well, reservoir connectivity, background fracture conductivity, and matrix contribution to flow. The pressure-buildup interpretation after the extended well test on Well 4 suggested that permeability was deteriorating rapidly with pressure depletion. A major uncertainty remained regarding the efficiency of bullhead stimulation in the openhole completion of Well 4 because the length of the open section was well over 600 m. Therefore, Well 5 was designed for a cemented liner completion. In total, seven subsurface targets were identified for perforation, stimulation, and well testing. Zones stimulated using the same technique would be grouped for a short individual cleanup-flow test. Afterwards, all groups would be commingled in a final well test. Selecting and Preparing Stimulation Designs for the Acid and Proppant Zones Considering the subsurface uncertainties and based on the stimulation and test results from previous wells, it was decided that the project should have a loss-zone-characteristics-driven stimulation concept (i. e. , combination of proppant and acid stimulations to cater to a wide range of subsurface scenarios). Proppant stimulations would be attempted only in minor- or medium-loss zones.