This paper describes the methods developed for testing and qualification of novel lightweight drilling fluids (foams, glass-bead fluids) using a unique pilot-scale test facility (PSTF). The performance criteria included fluid stability, rheology, pressure transmission, and gas migration under downhole conditions. Test results demonstrating the methods developed are provided, along with the capabilities of the facility, custom fixtures, and equipment that were built to study the performance of these fluids.
A set of performance criteria and testing requirements were initially developed, which were then used to design and fabricate a novel pilot facility. The PSTF could generate downhole drilling conditions of 7,500 psig and temperatures above 300°F. Three custom-instrumented test articles were built to simulate wellbore geometry; one 10-ft long and one 18-ft long, both with a 2.62-in ID. The third article had a 10-ft long 6-in × 4-in annulus, with the eccentric internal pipe capable of 100-rpm rotation to mimic the drill string. The test articles could incline up to 45° to simulate deviated wells. Gas could be injected, and its migration rate measured in static and countercurrent flow using a video camera with full-bore sight glass, and gamma-ray densitometers. Dedicated sections for foam generation, measuring density, rheology, pressure transmission, and fluid sampling and imaging were provided.
Upon commissioning of the PSTF, a 1 1/2 year test program was successfully carried out using lightweight foams and hollow glass-bead fluids. Due to the novel nature of the tests, best practices and procedures were developed through experimentation to quantify static and dynamic fluid stability, gas migration, foam generation techniques, fluid imaging and characterization, pressure transmission, and rheology. A variety of measurement techniques and instrumentation were trialed in the test articles to determine the best methods for tracking gas migration. Experiments in the test articles yielded a large amount of performance data, including fluid stability over time at different temperature and pressure conditions, the impact of drill string rotation on fluid stability, migration velocities of gas bubbles (i.e., gas kicks) within the drilling fluids at stagnant and countercurrent flow conditions, and the impact of drill string rotation. Pressure transmission speeds were measured in the foam with varying gas fractions. Example datasets from the testing program are provided, along with detailed descriptions of the test methods.
The methods and test facility used to study lightweight drilling fluids are unique to the authors’ knowledge. For the first time, drilling fluids were analyzed in an annulus with a rotating pipe at downhole conditions at a pilot scale, and fluid stability along with gas migration were studied. These provide for rigorous testing of lightweight drilling fluids; the application of these fluids is expected to increase with declining reservoir pressures in oil and gas fields.
