Recovery factor (RF) is one of the most important parameters for economic justification in the petroleum industry. The RF is calculated from the ratio between expected ultimate recovery (EUR) and original oil-in-place (OOIP). However, many uncertainties exist in OOIP estimations, mainly due to inaccurate information about reservoir drainage area. In unconventional reservoirs, the estimations of drainage area and OOIP are more complicated due to unknown factors such as fracture networks in the system, reservoir pressures, pore volumes, and in situ hydrocarbon properties.
We analyze here a case study of the Bakken Formation where the RF is still unclear and not frequently reported. RF for the Bakken formation from previous studies ranged from 0.7 – 50% (Price, 1984; Bohrer et al., 2008) derived from different approaches. In this paper, we use the material balance equation (MBE) approach to determine the deterministic (single value) and probabilistic (distribution) RF values in the Bakken Formation (the Antelope, Sanish and Parshall fields). We also perform a sensitivity analysis for the input parameters in our MBE approach.From the actual well production data, the EUR can be estimated from decline curve analysis (DCA) with knowledge of abandonment rate data. The DCA technique based on history data is used as forecast model for predicting the EUR and remaining volume. Once the RF and EUR are calculated, the OOIP value can be evaluated as the ratio between EUR and RF.
In the studied fields, RF result of the Parshall Field is highest because the produced gas-oil ratio is lower than the other two fields. From the EUR results, the high EUR area in the Antelope Field is located in the center of the field. For the Sanish and Parshall fields, high EUR areas are located in the east and west side, respectively. In addition, it appears that EUR has a direct relationship to the hydrocarbon pore volume per area (HCPV/area) distribution. For each field, the results of OOIP per well from the ratio between EUR and RF are presented, and these data can be compared with the volumetric OOIP for proper well spacing plan of future projects.