Proper test and evaluation methodology for reservoir drill-in fluids is an important step in the construction design of a producing wellbore. Drill-in fluids are specifically designed to meet drilling and completion objectives, simplify reservoir cleanup and maximize production rates in open hole completions. New protocols developed by drilling fluid and completion laboratories have proven to be effective tools for evaluating fluid designs that optimize production.
Drill-in fluid selection begins with an initial screening process taking into account qualifying issues such as environmental acceptability, reservoir temperature, physical limitations, and chemical compatibility between the fluid and the reservoir. The final fluid formulation should minimize formation damage, fluid leakoff rates, breakout pressures, and wellbore cleanup of drill-in and completion operations, thus maximizing the productivity of the well.
Formation damage risks can be minimized if laboratory testing procedures parallel specific reservoir conditions. Case studies confirm that a methodology based on laboratory data that incorporates drilling fluid, completion, and workover operation protocols is effective in lowering completion costs and increasing production.
Today, drilling and reservoir engineers are engaged in dialogue for the purpose of utilizing drilling and completion techniques that match reservoir needs. While a focus on up-hole drilling programs remains, operators are placing increased emphasis on reservoir entry techniques. One reason for this change is the growing number of extended reach, sidetracks, and horizontal wells with open hole completions. The popularity of these procedures has led to a rise of specialized fluids systems known as reservoir drill-in fluids.
An optimally designed drill-in fluid should:
conform to acceptable health, safety, and environmental standards
retain desirable drilling fluid properties
bridge all exposed pore openings with a specially sized material
deposit a non-damaging filter cake that is easily and effectively removed by initial production.
Selecting the best reservoir drill-in fluid for a specific application begins with a dependable screening process followed by critical laboratory testing. Final selection is based on reliable data predicting environmental acceptability, drilling fluid performance, effective filter cake removal, and maximum production rates.
Currently, there are no industry standards in place for drill-in fluid testing. It is the purpose of this paper to suggest and describe laboratory procedures and selection methods designed to evaluate drill-in fluid systems for reservoirs that will utilize open hole completions. Not every test is detailed, only those providing evidence of both drilling and completion performance from a single fluid. In addition, well histories incorporating drill-in fluid screening and testing and their relationship to the fluid's field performance will be reviewed.
Improved drill-in fluid designs are directly related to an understanding of the causes of formation damage. Extensive literature is available detailing the various mechanisms responsible for reservoir impairment. Those directly related to drill-in fluids include:
filtrate/formation fluid incompatibility
solids or whole mud invasion