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Title: Well test analysis of shale gas wells
Author: Kostyleva, Irina
ISNI:       0000 0004 7659 1126
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2019
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Shale gas reservoirs have received much attention in the last ten years with the development of stimulation techniques, namely massive hydraulic fracturing of horizontal wells. However, the processes taking place in these reservoirs during stimulation and production phases are not yet fully understood. Much of the research interest has been concentrated on decline curve analysis methods (for example, Stright and Gordon (1983)[96]), pore-scale description or forward modelling of pressure responses with the inclusion of pressure-dependent properties. The true challenge lies in explaining field data and understanding the dominating physical processes, and in particular permeability reduction, due to the fracture closures or conductivity impairments. The present research is concerned with analysis of field dynamic data of shale gas wells. A conceptual model was proposed and initially matched with two years of data from a number of Haynesville wells. Then three more years of production data were acquired, and the model was able to match the additional data. The model assumes three successive mechanisms governing well productivity deterioration: initially the matrix permeability declines, then the fracture conductivity is reduced, and finally, further latetime well productivity decline occurs which can be matched by introducing a pseudo-skin effect increasing linearly with time. These successive behaviours can be identified on a normalised rate versus cumulative production plot, where they are characterized by straight lines with distinct slopes. The conceptual model was represented in a 3D simulator by a single repetitive element corresponding to a section of a horizontal well with a single vertical fracture confined between the no-flow boundaries resulting from fracture interferences. The repetitive element includes two regions: a network of induced fractures around the well with enhanced permeability and an outer unfractured matrix with permeability values in the nano Darcy range. Permeability declines with pressure in both regions, according to a relationship based on core measurements in the matrix, and determined by history matching in the inner fractured region. Further productivity reduction at later times in some wells is accounted for with a pseudo-skin obtained by history matching. The direct application of conventional well test analysis and deconvolution methods to shale gas field data is not possible as rock properties are pressure-dependent and the problem is non-linear. Pseudo-time and pseudo-pressure functions were used to linearise the diffusivity equation, using the pressure-dependent properties obtained from history matching. Deconvolving them provided the derivative shape consistent with the responses expected from a multi-fractured horizontal well, thus providing an additional justification of the proposed approach and of the validity of the conceptual model developed in this study.
Supervisor: Gringarten, Alain Sponsor: Imperial College London
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral