This paper describes first results of an extensive laboratory research program aiming at identifying potential additives for bentonite suspensions which would extend their use at high temperatures by maintaining or even improving the rheological and the filtration characteristics of these fluids.
Five different Grrek lignites have been tested, at 3% wt. concentration, together with a commercial lignite product, used as reference. API filter press data and rheological data showed that thermal aging of bentonite suspensions results in deterioration of rheological and filtration properties while lignite addition was very beneficial.
Static filtrations tests against Berea sandstones have been performed using a core holder and the various mud formulations. Pressure ports along the core holder allowed determination of permeability evolution during testing of the formed cake as well as of the core itself. A backflash arrangement allowed for determination of permeability recovery.
Thermally aged muds reduced significantly core permeability and gave recovery rates around 50%. Three of the lignite samples, have maintained original core permeability giving also very high recovery rates. Some of the lignite samples, including the commercial one, did not perform as well and also gave permeability recoveries between 10 to 80%. Thus, static filtration against cores allows for differentiation of well performing lignite samples not only in API filter press tests but also in core permeability experiments.
Introduction
Drilling fluids have many functions such as to lubricate the bit, to transfer cuttings to surface, to protect producing formations by forming an external and internal filter cake and to apply overbalance to keep formation fluids into the formation 1. Various additives are thus used, in particular with water based drilling fluids, to aid the fluids to perform these functions, with bentonite being the major ingredient due to its ability to offer superb properties at low cost, providing in particular, very good rheological properties and filtration characteristics which minimize formation damage.
Formation damage by drilling fluids has long been recognized 2–4. The parameters affecting the performance of drilling fluids with respect to formation damage has thus been studied extensively, both experimentally 5–10 and theoretically 11–14. These factors are solid size, overbalance pressure, mud filtrate invasion, additive concentration and type, and downhole temperature.
There is a continuous and increasing interest for oil and gas resources in ever increasing depths requiring the drilling of very deep wells where excessively high pressures and temperatures (HPHT) are encountered, with temperatures in excess of 3000F (1500C) and wellhead pressures in excess of 10000 psig (690 bars)15,16.
Drilling fluid research has produced a range of drilling fluids that are capable of performing these formidable tasks in these difficult environments. However, it is well known that above about 120°C colloidal systems composed of bentonite begin to thicken considerably 1,17. These fluids may remain exposed to high temperatures for long periods where they form strong gels, imparting thus excessive pressure drop when flowing. In many situations, oil based muds are used because they are stable at these high temperatures but environmental, logistical and cost considerations limit the use of oil-based fluids. Therefore, water-based drilling fluids containing bentonite are often used in these difficult environments. Various additives are then added to overcome these gelling tendencies. Lignites have been used successfully, with or without metal complexes 1 because of their high thermal stability. Recently, extensive work has been performed to study the use of Greek lignites as potential additives for water-bentonite suspensions with very promising results, sometimes even better than commercial lignite additives, both for improving rheological properties 18–19 and for improving filtration characteristics of drilling fluids 20–21.
