The basal clastic sand (BCS) unit is derived from a granitic basement and forms the lower part of the Mumbai High field. Oil indicators in unconventional reservoirs, such as basement and BCS, were explored here before 1987; however, these reservoirs were not targeted for more than two decades after drilling the first exploratory well in 1989. Huge potential in BCS resources remained untapped, and monetizing these resources became possible because of extensive hydraulic fracturing design optimization for these layers. Previously, acid stimulation treatment failed to provide any improvement in the BCS reservoir. Because BCS is derived from a granitic basement and contains clay minerals (kaolinite and chlorite), heavy minerals, siderite, pyrite, hematite, etc., it is difficult to obtain gains using acid stimulation because of poor leakoff and associated reaction kinetics. However, stimulation using hydraulic fracturing with proppants proves to be the ultimate productivity enhancement tool and is the prudent alternative. The first hydraulic fracturing attempt in BCS was performed at Well B in 2013 and was unsuccessful because proppant placement and admittance are extremely difficult in these layers. High net pressure and complex branch growth were identified to be the core causes of premature screenout in this layer. Post-treatment pressure evaluation indicated propagation of short fissures and fractures leading to a complex fracture plane that reduced overall fracture conductivity. Subsequently, Well A was diverted from the original location, completed in the BCS reservoir, and selected as a candidate for proppant fracturing. The stimulation strategy was designed to meet stimulation challenges of the BCS formation. Perforation designs were revised to reduce near-wellbore tortuosity and perforation friction. After perforating, the well was treated with an acetic acid cushion against the target zone. A new fracturing treatment design based on slug and sweep, where the slug stages were increased, was used to control excessive near-wellbore complex fracture growth. Aggressive pumping rates and high conductivity proppant size and concentration were designed to help increase stimulation efficiency. These unconventional modifications aided successful placement of the fracture plane in the BCS reservoir in Well A. Well A initially produced 202 BOPD; however, production declined because of the tight nature of the BCS reservoir. Later, the well produced 100 BOPD with gas lift. After hydraulic fracturing treatment for this well was successfully performed, as per the modified design, the production increased to 1,580 BLPD with 100% oil and no artificial lift using a 1/2-in. choke. This paper highlights design considerations, execution results, and post-treatment evaluation of this extremely challenging BCS volcanic rock and can be viewed as a best practice for addressing stimulation challenges in similar volcanic reservoirs in other fields.

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The Daman marginal field is a prolific gas-producing clastic field with highly unconsolidated Paleo-Miocene sandstone formations and a wide variety of lithologies across multistack sand layers. As such, high-rate water packs (HRWPs) are the ideal completion method in many Mumbai fields. Because multistack reservoirs require good zonal isolation, and to prevent crossflow between reservoirs with different pressure regimes, multistack sand exclusion (MSSE) methodology was selected for primary well completions with minimum rig time and a high degree of treatment placement accuracy. From an operational standpoint, exploiting these layers using this method means more control points can be achieved across these heterogeneous layers, and the MSSE completion is ideal for multiple applications in a shorter period, helping sustain sand-circumscribed gas production from these unconsolidated layers. During the design phase, grain-size distributions and core study defined the sand range from generally clean, coarse, and sorted to poorly sorted, with high-fines content and clay rich. To address the unique challenges of deep offshore operations, formation technical difficulties, high-stakes economics, and the significant untapped potential from these Daman sands, the MSSE approach was designed and implemented in this field. Historically, for multistack wells, an HRWP is performed zone by zone whereby the process of sump packer installation, perforation run, deburr run, screen assembly installation, and pumping is repeated for each zone. In Well A, the MSSE system was applied without any repetition and all in one phase. All layers were perforated and positively isolated. Each interval was individually opened for the HRWP treatment using a low-friction low-residue carrier fluid. Using a high-packing-factor proppant at a higher rate, the well was treated sequentially from the bottom of the interval to the top. Many marginal fields in this basin have become uneconomical because of the high cost and complexity of sand control methodology. Therefore, reducing costs and time becomes vital to help ensure economic viability, as well as achieving significant operational efficiencies. Additionally, reducing near-wellbore (NWB) mechanical skin and ensuring good productivity from the reservoir are among the major solutions when implementing an MSSE completion. The methodology adopted significantly helped reduce expenditures by standardizing completion design, simplifying the core complexity, and enhancing overall reliability and operational efficiency. The optimized engineering workflow was fit for purpose, rather than the conventional “cookie-cutter” method to address sanding propensity in this field. This paper discusses the cutting-edge MSSE completion systems that focused on downhole completion and modifications for pumping operations. Additionally, the paper reviews challenges addressed during this campaign, workflow adapted, detailed strategy success factors, and positive results obtained during evaluation. This has helped reduce potential risks and improve reliability and performance, which can act as best practices and can be applied within similar fields.

Western offshore fields in the South and East Asia region are depleted and require frequent stimulation. To efficiently deliver stimulation treatments, a supply vessel has been converted into a well stimulation vessel with limited well flowback capability. This is one of only a few vessels in the world that has a zero-discharge flowback facility with certification by industry-recognized external organizations. Developing the vessel and operations plans required a systematic approach to continuous improvement. The process involved identifying the criteria for efficiency and effectiveness for an offshore stimulation vessel and building in continuous improvement methodology with a focus on visual order, organization, safety, and standardization of operations. A lean management methodology was applied to systematically analyze and improve operational process efficiency in relation to organization, orderliness, cleanliness, adherence, and self-discipline. A series of checklists were introduced to enable procedural adherence with monthly self-evaluations to identify areas of improvement and develop remedial work plans. This paper describes the vessel conversion and installation process as well as the lean approach and health, safety, and the environment (HSE) requirements. The vessel has been used for a variety of stimulation jobs such as proppant fracturing, sand control pumping, matrix stimulation, nitrogen pumping, and limited flowback. These stimulation jobs have been achieved with no recordable injuries and zero nonproductive time by introducing operating procedures, training crew members in those procedures, and monitoring adherence. Systematic efficiency improvements related to materials and procedures led to a 23% reduction in onboard crew size, thereby reducing the risk exposure to personnel. Vessel repair and refabricating expenditures have been reduced by 53%. This vessel has delivered an average of 40 jobs per month, achieving a peak job frequency of 64 jobs per month and treating more than 1,000 wells to date. This paper provides an overview of the practices developed and lessons learned for operating an offshore stimulation vessel, including process flow charts with trackers and checklists designed to improve operational efficiency.