Use of Parallel Compositional Simulation to Investigate Productivity Improvement Options for a Retrograde-Gas-Condensate Field: A Case Study

Abstract

This multi-disciplinary study involved the use of parallel simulation using the same grid size as the geological model. Various production technology options for the remaining wells in a field development were investigated and the benefits of gas cycling were examined. A geological model covering an area of 40 km2 was simulated using two approaches; one using 3 levels of nested LGR's and the other using an areal upscaling, both had nearly the same number of active grid cells. The 12-component Equation-of-State ("EoS") was substituted by 5-component to optimise the run speed of the simulation, 12-component was used for investigating gas cycling. The final engineering model had 320,000 active cells and completed 1.5-year history-matching runs in 1 hr 27 min using 16 parallel 400MHz processors with 1GB RAM each. Both methods worked well and the history was matched with minor changes to the geological model. The primary match parameters were the relative permeability curves since relative permeabilities from special core analysis at reservoir conditions were not available. Correlations from the literature were used for the original relative permeability curves. These were modified to capture the effects of near-well velocity stripping by ‘straightening’ the curves and moving the end points. Through a combination of local grid refinement (LGR) and relative permeability modifications, the effects of the condensate dropout near wellbore and the transient effects were captured. These effects reduce the wells' productivity by 40% in the first weeks of production. In the prediction phase the full field model was used to examine, holistically, the effects and interaction of various options chosen by analytical screening. The main solutions compared were high angle (near horizontal) reservoir penetrations and hydraulically fractured wells. Hydraulic fractures were modelled using LGR to capture the behaviour of the condensate in the fractures. The high angle wells were surrounded by LGR. A large number of prediction cases were run, optimising the development scenario using stochastically derived drilling schedules. Considerations of capital allocation led to the final recommendation from the group of optimal solutions. Introduction The Field consists of two main areas; the subsea gathering centre producing from two layers, and the platform area producing from a further three main layers. The wells in the platform area suffer from a considerable loss of productivity, up to 40%, during the first few months of their producing lives. This loss is attributable to the transient effects and the formation of a condensate bank around the wellbore. At the beginning of the study 14 platform wells were on production; two had been hydraulically fractured and one high angle well (HAW) had been drilled. The fractured wells had not been produced due to facilities constraints and the HAW expressed lower productivity than expected. The subsea area was not included in the study as it is of generally better reservoir quality and consequently does not suffer from the loss of productivity in the early production life. The objective of the study was to develop productivity improvement solutions for four type wells representing the Platform Area. The well types are as follows:the main area: good permeability, three layers; no gas water contactwestern well: low permeability, one layer, far away from the platformnorthern well: two main layers, low permeability, overlaying a high pressure shaleeastern well: one better layer; one lower quality layer with GWC. Previous work had concentrated on well level solutions; in this study the overall impact of the solutions on the field was investigated. Before embarking on massive reservoir simulation, an analytical screening study was performed to identify the future well types. The idea was to perform minimum simulation runs to optimise the field performance.

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Application of Build-Up Transient Pressure Analysis to Well Deliverability Forecasting in Gas Condensate Reservoirs Using Single-Phase and Two-Phase Pseudo-Pressures

“…This creates three different saturation zones around the well. Close to the wellbore, high condensate saturation reduces the effective permeability to gas, resulting in severe well productivity decline [1][2][3][4][5] . This decline is reduced at high gas rates and/or low capillary forces, because condensate saturation is lowered in the immediate vicinity of the wellbore, resulting in a corresponding increase in the gas relative permeability.…”