This paper discusses the systematic design and development of high performance water-based muds and provides insight into the unique chemistry and inhibition characteristics of various amine inhibitors. Equally important, performance correlations of inhibitive systems in laboratory testing as compared to state-of-the-art inhibitive systems are included.
Invert emulsion drilling fluids have long been effective in drilling reactive shale. Developing high performance (highly inhibitive) water-based drilling fluid that would perform like invert emulsion drilling fluid has long been cited as the ultimate technical goal of the drilling industry. Progressive development of inhibitive water-based drilling fluids based on amine chemistry has made some impact on reaching this goal. Amine-based inhibitive drilling fluids have steadily gained popularity with service and oil companies.
However, these fluids have not always been completely successful in inhibiting the hydration of highly water-sensitive clays. The short-comings are particularly evident when drilling highly complicated and reactive shale formations. Keeping this in mind, an innovative highly inhibitive water-based drilling fluid has been systematically designed with the performance characteristics of oil-based muds.
The newly developed high performance water-based mud (HPWBM) comprises a unique polymeric amine shale intercalator for shale inhibition, an amphoteric polymeric shale encapsulator, a high performance lubricant/antiaccretion agent and a specialized fluid-loss additive. The newly developed HPWBM performed like an oil-based mud in laboratory testing as well as in offset wells using invert emulsion drilling fluids (OBM) due to highly complicated and reactive shale formations.
To address the drilling problems associated with shale instability various non-aqueous drilling fluids (NADF) such as mineral oils, saturated and unsaturated poly alpha olefins and esters have been developed and utilized in the field.1–4 Along with the shale stability benefits of these NADF, various other benefits like lubricity, temperature stability, and anti-accretion are attributed to NADF. These distinguished benefits of NADF usually are cited as the technical goal of an ultimate HPWBM.
Along with those advantages, NADF have disadvantages, such as high cost, environmental limitations, disposal problems, health and safety issues and detrimental effects on the drilling and completion of the pay zone. Consequently, a water-based drilling fluid which performs like an oil-based mud has been an ambitious goal of the drilling industry. Two characteristics of the HPWBM have been identified that contribute significantly to performance of the drilling fluid - shale stabilization and lubricity properties. These OBM characteristics serve as design targets to many researchers of aqueous-based systems striving to achieve the performance of OBM system when using a WBM.5–8
When water-sensitive shale is exposed to conventional water-based drilling fluids, shale has an immediate tendency to take up water from the drilling fluid. Depending upon the chemical characteristics of the shale, this can result in a rapid swelling or dispersion of the shale. Consequently typical problems such as bit-balling, disintegration of cuttings, borehole wash-out, high torque and drag, and stuck pipe are often encountered as a result of water adsorption by water sensitive shale.9–10
For more than the past five decades, various chemicals have been used for inhibition of water-sensitive shales. Among the earliest and most widely used method relies on the use of high concentration of salts such as potassium chloride, sodium chloride and divalent brines. These salts through a variety of mechanisms might be claimed to somehow retard swelling. The early development of the shale inhibition fluids included sodium chloride/starch muds,11 silicate muds,12 lime-muds and calcium sulfate-based gyp muds.