--> Geomechanics Aided Solutions For Improving Drilling And Completions By Integrating Acoustics Measurements For The First Time In Mandapeta Field, Onshore Kg-Pg Basin, India

AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment

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Geomechanics Aided Solutions For Improving Drilling And Completions By Integrating Acoustics Measurements For The First Time In Mandapeta Field, Onshore Kg-Pg Basin, India

Abstract

Krishna Godavari Basin is a peri-cratonic passive margin basin on the east coast of India. The basin’s characteristic feature is its en-echelon horst and graben structure which is filled with sediments of Permian to recent age. KG basin is considered to be one of the most promising petroliferous basin on India. Mandapeta field is located in KG basin with proven gas reserves in Mandapeta formations of Triassic age. Oil and Natural Gas (ONGC) ltd is operating and undergoing a development campaign in this field recently where they have witnessed high NPT due to various drilling challenges such as mud loss, tight hole, stuck pipe etc. Nonetheless, poor drilling condition led to poor borehole condition and hence poor logging condition. Most of the problems are faced in Raghavapuram, Gollapilli and Mandapeta formations. To minimize these drilling related risks, an integrated customized solution based on an iterative mechanical earth modelling and wellbore stability analysis approach utilizing acoustic outputs and drilling events. The geomechanical model was consistent with the failure seen in multi-arm caliper. Geomechanical analysis helped client to identify the problems and design the solution to mitigate the same in advance; like- significant increase in pore pressure upto 1.46gm/cc in Raghavapuram to Mandapeta formations; operate within a narrower mud weight window of 0.2gm/cc while drilling through the reservoir; thus, requires proper mud weight design and isolation of different pressure regimes with improvised casing policy at the same time. Three wells are analyzed in the area utilizing inputs from sonic and other wireline measurements. Optimum mud weights and casing policy design were recommended for future wells in the field. Integration of geomechanical analysis with sonic and other open hole logs led to identification of Type-II overpressure from Raghavapuram shale onwards. Velocity versus effective stress (Bower’s plot) denoted that some overpressure points of Raghavapuram and formations below lie in unloading curve which clearly indicated Type-II overpressure in those formations which can be due to faulting, fluid migration, clay diagenesis etc. In addition to this, density versus velocity plot (Hoseni, 2004) shows data from these two formations lying parallel to velocity axis which indicates clay diagenesis. Chemical diagenesis refers to the chemical alteration of minerals by geologic processes and is thought to be a principle cause of abnormal pressure (McClure, 1983). For example, temperature increases during rock deposition may cause Montmorillonite family clays to dehydrate and turn into illite (Powers, 1967). Water that was formerly bound to the Montmorillonite is thus released to occupy pore space. Because free water occupies more space than bound water, it becomes over pressured if held in by a low-permeability seal. This cause of overpressure has been identified for the first time in this field. Apart from drilling optimization, geomechanical study had also helped in completion quality estimation for selection of good zones to perforate and fracture in the reservoir formation. The zones with high minimum horizontal stress barrier were identified as good zones for perforation. Completion quality analysis suggested requirement of multi-stage fracturing in sand intervals of Mandapeta formation.