William W. Weiss

Producing GOR Used to Predict Permeability Distribution in a Tight Heterogeneous Reservoir

Production from tight reservoirs can yield flow pressures around the wellbores that are below the bubblepoint. The accumulation of free gas around the wellbores results in a rapid increase in the producing gas-oil ratio (GOR). This paper investigates the sensitivity of depletion-drive GOR to variations in permeability. A public domain, black oil simulator was modified to automatically estimate the permeability from GOR histories. The method presented in this paper is especially useful when interpretable pressure transient tests are not available and can be adapted to existing simulators without changing the core code. A multi-objective, simultaneous bisection algorithm was developed to provide automatic GOR history match and to predict permeability distributions. Following each iterative simulation run, the permeability-searching intervals for all of the wells were simultaneously reduced by half and the permeability distribution map was updated. The updated searching intervals are uniquely determined because the permeability-GOR correlation is monotonic. Usually a satisfactory match is obtained within 10 simulation runs, which is much more efficient than the gradient or stochastic methods typically used for an automatic history matching procedure. This method was applied to field data from the South Bisti Gallup Pool. Well communication tests have shown a strong permeability preference for a southwest-northeast orientation in the pools two tight pay zones. The automatic GOR match was applied to the 14-year depletion history of 52 wells. First, the ratio of K g /K o was estimated from the total field GOR match. Then, the local variation of permeability was determined according to the GOR history of the wells. Two perpendicular- oriented permeability distributions were obtained. This new method should be valuable for other low permeability reservoirs.