Reevaluation of Hydrocarbon Reserves in Old Fields Through Cased-Hole Interpretation: A New Approach

Abstract

Abstract The widely recognized importance of the re-evaluation of existing hydrocarbon fields has focused the attention of oil companies on the problem of cased hole logging problem of cased hole logging interpretation. Cased hole logs actually provide a lot of information. However, their response, particularly for the Density log, is heavily affected by the presence of casing and cement. For presence of casing and cement. For this reason, cased hole recordings are mainly utilized from a qualitative or semi-quantitative point of view by applying empirical corrections to the measured values. A new algorithm for the automatic correction of all the environmental effects has been developed for the Density log. It is based on the evaluation of the relative contribution of casing and cement to the bulk density measurement. Thence, also corrosion can between into account. Casing collars and other disturbances are corrected through a multiple linear regression technique. Neutron, Sonic and Gamma Ray logs undergo a calibration procedure and their reliability is accurately verified. A case study relevant to an old Italian gas field is presented. Introduction In the last years the re evaluation of recoverable reserves in old hydrocarbon fields has become one of the major targets of an oil company policy. policy. Many fields, mainly gas fields, were discovered in Italy in the Fifties and early Sixties in the Po valley. They have been producing for many years. The re-evaluation of the remaining reserves cannot be carried out using only the existing data. Most times, in fact, the available data for these fields are very scarce and of poor quality: few cores and old, low poor quality: few cores and old, low resolution logs. As a consequence, the need for a correct re-evaluation of the existing and already producing fields gave a pent role to cased hole log pent role to cased hole log recordings and interpretation. Cased hole logs interpretation is a hard task. Most logs are generally heavily influenced by the presence of casing and cement. In fact, cased hole recordings are commonly used from a qualitative point of view. P. 167