A Laboratory Drilling Mud Overbalance Formation Damage Study Utilising Cryogenic SEM Techniques

Abstract

Abstract Laboratory reservoir conditions flood tests were used to determine the clean up efficiency of a drilling mud that was applied at two different overbalance pressures. The test was designed to simulate, as closely as possible, the conditions occurring in the reservoir during the drilling operation. Relevant core material was used and the pore/overburden pressures, mud overbalance pressures, temperature and production drawdown pressures/rates applicable to field conditions were applied in the laboratory simulation. Drilling mud filtrate volume loss versus time measurements were made at reservoir conditions using sophisticated pumps. All wetted parts of the rig were constructed of hastelloy C276 alloy to avoid corrosion artifacts interfering with the test. Permeability measurements were made prior to mud application and again at the end of the flood test analysis to determine the level of any formation damage. In order to determine the nature of the formation damage mechanisms, geological techniques which included dry and cryogenic SEM and thin section analyses were undertaken. These techniques were applied to the plug trims prior to testing (untreated samples) to determine the natural state of the rock, paying particular attention to the clay mineral and cement types, morphologies and distributions. The plug samples were examined at the end of testing (treated samples) and any variations noted which would highlight the damage mechanisms. The dry SEM and thin section analyses were used to determine solid damage mechanisms such as clay fines migration, scale precipitation and drilling mud solids invasion. The cryogenic SEM analyses were used to examine any fluid damage mechanisms such as wettablity alteration, microemulsion formation, water/oil blocks or fluid retention. A new technique of cryogenic SEM using EDX (Energy Dispersive X-Ray) analysis was used for X-ray mapping of the remnant mud bodies to determine solid and fluid distributions. Introduction The objective of the laboratory analysis performed was to determine the effect of high over balance during drilling followed by simulated return to production on the productivity from a particular lithology in an oil well. Any damage mechanisms and there impact on productivity were to be fully investigated and implications for future drilling and completion highlighted. A series of reservoir conditions core flood tests were designed to measure the permeability changes caused by different well conditions1. The core flood tests were designed to simulate as closely as possible the real wellbore conditions. Thorough sample evaluation was used to identify all damage mechanisms and reasons for measured permeability changes. Combinations of standard and innovative sample evaluation techniques were used to examine the core plug samples after the flood tests and the external bodies developed during mud application and remaining after simulated return to production. Core Flood Test Procedures Sample Selection and Preparation. A total of six one inch diameter core plug samples were cut from a wax preserved whole core sample. The plug samples were cleaned using a warm submerged soxhlet cleaning technique and dried in a 60°C dry oven. End face debris caused during trimming of the plug samples was removed using the acetate peel technique2. Base parameters of porosity, permeability and grain density were measured. The plug samples were C. T. scanned in order to view any heterogeneity or anomalies which might adversely affect the core flood testing. Three samples were selected to be representative of the target lithology and with relatively similar base parameters. The three samples were initially saturated 100% to simulated formation water (see below) and then spun to irreducible formation water saturation using an ultracentrifuge. The ultracentrifuge technique is used in order to obtain a consistent, repeatable saturation relatively quickly without mobilising fines within the plug samples. Sample Selection and Preparation. A total of six one inch diameter core plug samples were cut from a wax preserved whole core sample. The plug samples were cleaned using a warm submerged soxhlet cleaning technique and dried in a 60°C dry oven. End face debris caused during trimming of the plug samples was removed using the acetate peel technique2. Base parameters of porosity, permeability and grain density were measured. The plug samples were C.T. scanned in order to view any heterogeneity or anomalies which might adversely affect the core flood testing. Three samples were selected to be representative of the target lithology and with relatively similar base parameters. The three samples were initially saturated 100% to simulated formation water (see below) and then spun to irreducible formation water saturation using an ultracentrifuge. The ultracentrifuge technique is used in order to obtain a consistent, repeatable saturation relatively quickly without mobilising fines within the plug samples.