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Delaware Basin GOR and Production Forecasting


In tight-oil reservoirs such as in the Delaware Basin, pressure will eventually drop below the bubble-point pressure, thus generating two-phase gas-oil flow in the reservoir. Increasing gas-oil-ratios (or GORs) have been cited as potentially steepening oil production decline in wells and reducing ultimate oil recovery. Operators have cited large differences between predicted and observed GORs, and have found large variability in observed GORs, even in adjacent wells. Fear of the effect of a rising GOR on the estimated ultimate recovery (EUR) of oil indicates the need for a systematic study of GOR as a function of spatial location, related reservoir and fluid characteristics, and operational controls.

The objectives of this study are to develop models and methodologies:

(1) To forecast oil rates and gas-oil ratios (GORs) and the estimated ultimate recovery (EUR) for oil wells in the Delaware Basin;

(2) To determine geological controls on the reservoir fluid composition and gas-oil ration using basin modeling, thermal history and nanotechnology;

(3) To estimate oil and gas recovery factors for these wells as functions of formation characteristics, fluid properties, and operational controls; and

(4) To reliably quantify the uncertainty in these forecasts and estimates.

New workflows have been developed to integrate various components of study, particularly focused on the objectives given above.

Fundamental to this study is determination of geological controls (e.g., source, thermal history, maturity, pore size, pore type) on reservoir fluid properties — and thus on GOR, and on oil and gas recovery. Also fundamental is the need to determine and model the governing physics of the PVT properties of these reservoir fluids as well as the fluid storage/flow in nanopores for the reservoirs in the Delaware Basin (in particular, the Avalon, Bone Spring, and Wolfcamp reservoirs). This modeling will lead to forecasting techniques that include:

(1) Rigorous "full-physics" compositional simulation (including nanopore storage and transport) for in-depth understanding of controlling physical processes;

(2) Less rigorous conventional compositional simulation with modifications to approximate the influence of nanopores;

(3) Analytical models similar to those in commercial rate-transient analysis software; and

(4) Production-decline models using only typical publicly-reported rate data or using measured rates and pressures.

Since development of the Delaware Basin is relatively immature, the results of this proposed study may have a significant positive impact on development plans and ultimately on EUR and NPV of wells completed in the basin.