Abstract

The Ratana Field is a gas condensate field located in the Potwar Basin of Pakistan, discovered in 1990. The field comprises several stacked reservoirs in a thrusted anticlinal fold with two main reservoir compartments, an eastern and a western, which are separated by the main thrusts, but appear to be in pressure continuity through a fracture network. Ratana-2 was the first well drilled in the western compartment which came on production from the Paleocene Patala Formation limestone reservoir and is a major producer in the field. Ratana-2 was subsequently deepened to the Jurassic Datta Formation but because of a stuck drill string in the hole all penetrated reservoirs (Chorgali/Sakesar, Patala, Lockhart and Datta) have been open to flow since November 2009. Well interventions are not therefore possible in Ratana-2 and production and reserves allocation based on limited dynamic data are thus very challenging. Surface-based parameters including water and gas composition analysis, wellhead shut-in pressures, surface production rates and other relevant data were used to perform production and reserves allocation by using a molar equation. The results of the molar equation predictions were cross-checked with the data from nearby well Ratana-4 which is also producing commingled from Paleocene and Jurassic age reservoirs since November 2015. Production logging surveys were acquired twice in over a period of six months and were consistent with the molar equation-derived formation-wise production allocation. Subsequently, the molar equation was used on the available surface data of Ratana-2 for the formation-wise production and reserves allocation and indicates for the first time that the Lockhart Formation as is a contributing reservoir in the Ratana Field and enabling the Lockhart Formation reserves to be booked after seven years of production.

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