Abstract
With exploration in harsh environments and consequent high pressure and temperature conditions, the calculation of reservoir properties has become complex and thus the changes in pressure transient response need to be understood and appreciated by taking appropriate challenging measures.
The paper deals with the various challenges arising when dealing with the drilling and testing of HPHT gas wells with Hydrogen sulfide and Carbon dioxide, located in Krishna Godavari (KG) Basin and the difficulties faced while executing it. The paper focuses on the experience while drilling the reservoir with a different mud program and mechanical failure caused by HPHT conditions & highly corrosive environment. The paper also highlights the preference of SOBM over WBM while drilling the reservoir section. It also describes the learning process as the exploratory well campaign progresses from one well to other. It briefs about the challenges while performing MDT as per the program in these high temperature environment. The paper briefs about the decision involved in selection of proper grade tubing, elastomer, packer, flowhead equipments, DST tools & explosives in this HPHT environment along with Hydrogen sulfide & Carbon Dioxide. In any gas well testing, exhaustive amount of data over the requisite period of time are necessary; data redundancy necessitates redressing of equipments. The biggest challenges faced by industry are high temperature rather than high pressures, so making metallurgy an important basis of consideration. It also highlights about the method followed during correlation of prospective zones using different logs.
The paper discusses the unexpected results and observations obtained during execution of our program and the lessons learnt from it.
Introduction
As a part of exploration campaign in Krishna Godavari basin in east coast of India, Gujarat State Petroleum Corporation led consortium drilled 4 wells which discovered very tight gas reservoir with average pressure of 12,000 psi & average recorded temperature of 360 oF therefore being classified as HPHT reservoir shown in Fig-1. This paper discusses the experience of four wells while drilling using a jackup rig with average water depth of 60 m.
Well-A was the first well of the series of 4 wells drilled. Well-B discovered and flowed gas from stratigraphy below the section encountered in Well-A. Well-C encountered same sands as of Well-A and discovered additional shallower sands not encountered in Well-A or Well-B due to erosion of section due to unconformity. The reservoir section is overlain by shale. The Well-A well was drilled in 6 sections as it was the first exploratory well while other three were drilled in 5 sections. These sections were 30??, 26??, 17 ½??, 12 ¼?? & 8 ½?? except in Well-A where a 6?? hole was also drilled. These sections were cased by 30??, 20??, 13 3/8??, 9 5/8?? and 7?? liner casings except in Well-A which also had a 5?? liner run. The reservoir section expected in 8 ½?? hole from seismic and log data was proved while drilling Well-A and appraised by wells Well-B, Well-C & D. Well-A & Well-B were drilled to the total depth using water based mud. The 12 ¼'' section of Well-C was drilled using WBM while 8 ½'' with SOBM. The shale sections & reservoir section in Well-D were drilled using synthetic oil based mud. The mud program was designed based on the pore pressure-LOT vs. depth chart shown in Fig-2.
