Abstract

Carbonate reservoirs hold 50 to 60% of the world's conventional hydrocarbons and are therefore of major economic significance. Solids production from poorly consolidated carbonate reservoir rocks is a common factor in many wells during production stages and especially in the later life of a reservoir under depletion conditions. While there are several well-estblished technuues to predict sand production in sandstone reservoirs, there are not many field-proven case studies on the applicability of these techniques for solids production assessment in carbonate rocks. In this study, we present a case study on the application of a widely-used sanding evaluation technique in a carbonate gas reservoir, offshore Vietnam.

We show production, core and well data from several production and appraisal wells in three nearby gas fields to investigate the cause of well productivity reduction in one well through a systematic geomechanical assessment of solids production. The findings are used to optimize gas production in existing wells, determine maximum solids-free drawdown for a planned infill well, and completion decisions for two new development wells to be drilled and completed in 2018 in a field with no production to date.

Rock mechanics tests conducted on several carbonate samples confirmed the low strength and high heterogeneity nature of the reservoir carbonates. Core-log calibration of rock mechanics paramters revealed reasonably strong correlations between rock strength and porosity logs which were used to generate continuous log-derrived rock strength profiles. Applicability of a well-established poro-elastic sanding predction model for carbonate reservoirs was investigated by a logical solids production analysis in a cased and perforated explorationwell and two open hole production wells. Production, well testsand fluid sampling data were used to calibrate and verify the solids production model.

Analysis showed no risks of solids production for the weakest reservoirs at initial and current reservoir pressures with the current applied drawdowns, and the drawdown can indeed increase in exsiting wells by another ~200 psi without the risk of solids production. The results however indicated that as the reservoir pressure reduces the solids-free drawdowndecreases rapidly and will approache zero when the reservoir pressure dropsby another600-800 psi. Therefore, solids control measures are essential to prevent solids production at later life of the field.

The results of the study and historical knowledge of the field reservoir and production were used for completion design of two vertical infill wells with the aim of increasing production from one of the major gas-producing blocks for Vietnam's domestic gas supply. The infill vertical wells will be completed open hole with external gravel pack and screen as the solids control measures.

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