Coring Unconsolidated Formation—Lower Fars: A Case Study

Abstract

Abstract Lower Fars is a shallow unconsolidated sandstone reservoir with high inter-granular porosity filled with heavy oil in southern part of Ratqa Field in Kuwait. The shallow depth (500' to 800'), friable sand laminated with shale and high viscosity heavy oil in pores have made coring this sand quite challenging. Last recovered core with rubber-sleeved core barrel in the eighties was only up to 60%. This paper describes how thoughtful mix-up of technology, innovative techniques and proper coordination by aligning all concerned has helped in meeting the challenge of coring unconsolidated sand and it's processing. Using low invasive core fluid, shorter core length, Aluminum inner core barrel, separate core bit to cut major sand / shale, full core catcher system, vertical slabbing at well mouth and on-site freezing have improved core recovery in excess of 85%. While low-solid content coring fluid with a pH of 9 resulted in low mud invasion, reduced WOB, ROP and SPM ensured fewer washouts during coring. Low abrasive core head with clam shell full closure core catcher produced good recovery. Core barrel length was reduced from standard 30' to 10', which was slabbed to 3' size keeping barrel vertically at well mouth. At well site it was frozen vertically with foam on top to minimise lateral movement and transported in freezer. At Core Lab it was kept frozen with dry ice, slabbed and plugged with liquid N2. It is expected that the obtained core plugs from similarly cored 6 wells shall lead to meaningful Routine and Special Core Analysis, which was suspected in old cores. This would help in developing the depositional geological model in conjunction with the image logs. Introduction The success of core analysis depends largely on how good the coring operation and well site core preservation are made. Conventional Coring in the unconsolidated formation is always problematic. The shallow sandstone reservoir filled with heavy oil in southern Ratqa Field of Kuwait is a challenge by itself. Earlier attempts in the eighties with rubber-sleeved core barrel had resulted in maximum recovery only up to 60%. When new wells were drilled as part of development plan, it was decided to core some early wells using modern coring technology. Low invasive core fluid, shorter core barrel length, Aluminum inner core barrel, separate core bit to cut major sand / shale and full core catcher system had helped in core recovery in excess of 85%. While low-solid content coring fluid resulted in low mud invasion, reduced WOB, ROP and SPM ensured fewer washouts during coring. Low abrasive core head with clam shell full closure core catcher resulted in better recovery. Core barrel length was reduced from standard 30' to 10', which was slabbed to 3' size keeping barrel vertically at well mouth. At well site it was frozen vertically with foam on top to minimise lateral movement before it was transferred to a chest freezer. The freezer was transported in running condition to Core Lab where it was kept frozen with dry ice for atleast 2 weeks. Core slabbing and plugging were performed using liquid N2. The obtained core plugs helped in meaningful Routine and Special Core Analysis, which was suspected in old cores. The good core recovery had also helped in characterization of depositional facies leading to the geological model in conjunction with the image logs. Unconsolidated sand coring- a new ball game The main areas of concern for unconsolidated formation coring are:minimising mud invasionusing proper coring assembly so as not to lose low strength rock capture during coringcore handling process so as not to jeopardize core preservation andnon-reactive preservation material.