Geomechanical Fracture Modelling Helps Renew Development of the Coevorden Zechstein Tight-Gas Reservoir

By

Taco den Bezemer¹, Karen Foster¹, Solenn Bettembourg², Stephen Bourne²

(1) Nederlandse Aardolie Maatschappij B.V, Assen, Netherlands (2) Shell International E&P B.V, Rijswijk, Netherlands

 The Coevorden Zechstein gas field consists of a large number of poorly connected, tilted fault blocks and the reservoir is interpreted to have been deposited in carbonate slope/deep water environment. Production from the Coevorden Zechstein wells has been highly variable and where good productivity occurs, it is attributed to the presence of fracture networks.

To predict the distribution of fracture networks, Poly3D, a deterministic geomechanical fracture modelling tool using boundary-element techniques, was chosen. This model computes the stress field related to slip on seismically visible faults based on estimates of the regional stress tensor. The main assumptions of the model - fracturing is related to strike-slip or normal faulting and the rock is isotropic, homogenous and linear elastic - seem reasonable for our setting in that no correlation has been found between folding (curvature) and fracturing, nor has a relationship between dolomitisation and fracturing been found. Additionaly, lithological variation across the field seems small given its depositional setting, and no strong diagenetic heterogeneity has been observed. Using the stress tensor distribution, calculated using Poly3D, brittle failure analysis was carried out to produce failure maps indicating possible areas of connected fracture systems. These failure maps in turn were used to fill the dual-permeability grid of Shell’s dynamic reservoir simulator.

History matching revealed that 70% of the wells can be matched using a specific set of brittle fracture parameters and fracture and matrix permeability. An attempt to history match the production behaviour using a random distribution of dual permeability or a much simpler model of fracturing failed. Similarly, no previous subsurface model has been able to withstand testing against dynamic data. Based upon this, the Poly3D methodology is facilitating further development of the Zechstein gas fields.