Abstract
An operator in India is developing the Vashishta and S1 gas fields, which are located at water depths between 250 and 700 m. The wells produce from thick sediments, ranging from Late Carboniferous to Pleistocene age. Because of the long producing reservoirs and large variations in sand grain sizes/permeability, premium screens with an enhanced gravel pack technique were used to complete the wells. This paper discusses the development and successful application of this integrated method.
A significant challenge in these fields is ensuring hydrocarbon flow through the complex, highly unpredictable, and very loose sand-producing intervals. The first conventional sand control completion attempt in the S1 field was performed in Well A in 2015, which later proved to be unsuccessful because of a near-wellbore (NWB) high net skin increase and a sharp decline in productivity. A significant amount of fines and the destabilization of high-clay-content formations were identified as the primary causes, along with drilling fluid damage in these reservoirs. Extensive core analysis, X-ray diffraction (XRD), and cation exchange capacity (CEC) evaluations were performed; these evaluations indicated that the water reaction with these clay minerals may have caused clay swelling, dispersion, and flocculation. Failure to address these effects in Well A led to formation softening and sloughing, resulting in significant permeability loss and NWB mud damage.
The sand control methodology was modified to address the unique challenges of deepwater operations, formation technical difficulties, high-stakes economics, and untapped potential from these reservoirs. Before the sand control technique was applied, an organic acid blend was introduced to help significantly reduce NWB damage. Particular attention was given to minimizing the destabilization of these high-clay-content formations. An ultralow molecular weight cationic organic polymer (PC-4) was spearheaded into the reservoirs to provide long-term water-sensitivity protection by rendering water-swelling clays insensitive to changes in water salinity; this process resulted in excellent sustained permeabilities. An advanced, cleaner hydroxyethylcellulose (HEC)-based polymer system was selected as the gravel pack carrier fluid, rather than the guar-based fluid used in Well A. The advanced HEC-based system achieved the highest regained permeability, as compared to other polymers, indicating the least formation damage.
The sand control completion using the enhanced gravel pack technique proved to be the ultimate productivity assurance tool and a prudent alternative for these formations. This technique is designed to access the significant potential of the Vashishta and S1 fields, which are free-gas fields with estimated reserves of 12920 MMscm and 10370 MMscm, respectively, and is equipped to meet the operator high targets. During the first five years, peak production is expected to reach 3.55 and 2.2 MMscm/d, respectively. This paper highlights design considerations, performance results, and post-treatment understanding of these extremely challenging formations and can be considered a best practice for addressing sand control challenges in similar reservoirs in other fields.
