Saturation Height Function in a Field Under Imbibition: A Case Study

摘要

This paper presents a case study on the reconciliation of the water saturation computed using electrical logs and a saturation height function when the field is under imbibition. It is noted that wireline log evaluations suggest that the reservoir system cannot be adequately described using primary drainage capillary pressure in the field, and structure restoration study supports that the reservoir is under the imbibition process. Gas in place (GIIP) has the largest impact on the development planning of this offshore gas field. GIIP is computed using the gas saturation, area and thickness of the reservoir making gas saturation a critical evaluation parameter. To compute the gas saturation accurately, typically, two sources of data, wireline logs and core measurements are integrated and reconciled. Wireline log based saturation is computed by using density, neutron porosity, resistivity as well as dedicated electrical core measurements, Archie’s exponents m and n. In addition, core based saturation is computed using saturation height functions established from capillary pressure measurements acquired using various techniques: Mercury Injection, Centrifuge and Porous Plate experiments. For imbibition conditions, special spontaneous and forced imbibition measurements were acquired on the core samples. These data sets represent two independent evaluations of the gas saturations. In this paper, an imbibition function is computed from drainage capillary pressure data from core measurements and reconciled with wireline log estimates in a systematic workflow. This workflow has been applied to the Ichthys field, Browse Basin, on the NW shelf of Australia for generating the pre-development drilling base case model. Overall, a good match is obtained between the saturation computed by the imbibition function and the wireline log interpretations, however mismatch is observed across some zones. Methods are proposed to clarify the origin of the mismatch and validate the resultant saturation whilst also serving as a guide for establishing a robust workflow to achieve these results.