Pressure Depletion Evaluation Behind Casing Using Pulsed Neutron Technology: An Application in a Multi-Layer Gas Reservoir in Eastern “Pianura Padana” Basin

Bartelucci, P. (Eni S.p.A.) | Borghi, M. (Eni S.p.A.) | Crottini, A. (Eni S.p.A.) | Galli, G. (Eni S.p.A.) | Pirrone, M. (Eni S.p.A.) | Rizzo, G. (Eni S.p.A.) | Nardiello, R. (Baker Hughes Inc. ) | Chace, D. (Baker Hughes Inc. ) | Kim, Y. (Baker Hughes Inc. ) | Zhang, Q. (Baker Hughes Inc. )



This paper presents a cased hole methodology for gas density and pressure calculation to quantify the gas depressurization in mature gas fields by use of Pulsed Neutron technology. The approach is based on an advanced interpretation of gas saturation behind casing and utilizes Monte Carlo reservoir characterization models based on different gas density responses.

The field application refers to an onshore multi-layer sand reservoir in the eastern Pianura Padana Basin producing biogenic gas since 1971. A strong decline of static bottom hole pressure (SBHP) has been recorded during the field life. The pressure drop is not constant and/or equal across the field gas levels and it may vary a lot between them. In many cases, the pressure of the single layers cannot be measured since two or more layers are completed to the same casing string.

The following proposed methodology allowed to quantify the different depletion degree of the main field levels for reservoir management and production optimization purposes. The first step of the analysis is to calculate the current reservoir gas saturation using commercial pulsed neutron ratio-based measurements. The following pressure depletion analysis could be applied if the original water saturation is unchanged and only a pressure drop has occurred. Capture Cross Section (Sigma) analysis is relatively insensitive to pressure depletion, but it can be used with confidence to confirm that the original water saturation has not changed. Assuming unchanged the open hole reservoir conditions in terms of reservoir saturation and pressure, if the current ratio derived gas saturation excesses the open hole saturation, then the gas density is iteratively reduced and the calculation repeated using new modeled responses. Depleted gas density and related reservoir pressure are finally determined when calculated cased hole gas saturation matches the original open hole gas saturation.