The Situation

The Operator was evaluating inhibitive water base alternatives to oil-base mud. M-I was requested to formulate a drilling fluid with inhibitive characteristics to minimize problems associated with highly reactive clays. Cutting and core samples provided by the Operator from the Panna Field were analyzed for mineralogy and cation exchange capacity (CEC_SHALE). The shales samples had a highly variable clay content; the dominant clays were smectite and kaolinite.

The shales were considered very reactive with 15-38% smectite and CEC_SHALE. 12-30 meq/100 g. Problems associated with drilling in this area using water base fluids include bit balling, tight hole and stuck pipe. Three basic inhibitive systems - GLYDRIL (glycol / potassium chloride), SILDRIL (silicate / potassium chloride) - and CALDRIL calcium chloride/polymer) – were compared with the conventional, low pH Desco system used by the Operator.

The Solution

A series of inhibition screening tests were performed, which included dispersion and swellmeter analysis to identify the fluid chemistry that would best address the problems associated with the shale reactivity. The most promising fluids were then tested in a down-hole simulation cell on core samples provided by the Operator. The silicate based system SILDRIL proved to be the best overall water-base mud with respect to inhibition.

The SILDRIL system is a salt/polymer system utilizing sodium silicate for improved inhibition. The primary inhibition is achieved with the addition of a unique “2.6 ratio” silicate (SiO₂:Na₂O ratio). The non-dispersed SILDRIL system was designed with 8% by weight KCl in the fluid phase. Results from the study convinced the Operator to use the SILDRIL system on an offshore well in the Arabian Sea (Bombay Area).

The Results

The Operator was able to drill the well safely to total depth, with no mud-related problems. The unique silicate chemistry provided chemical and physical stabilization of reactive shales and claystones. The properties were very stable throughout the interval with little treatment apart from volume maintenance. Cuttings integrity at the shale shakers was very good during drilling with the cuttings being well defined at the surface. The teeth marks from the PDC bit used to drill the interval could be seen on the cuttings at the surface indicating that the mud was performing an excellent job of inhibiting the formation. At no time during drilling did the solids build up in the fluid. The result was no whole mud dilution. The shakers, dressed with 110 and 150 mesh screens, handled the mud with flow rates up to 900 GPM. No whole mud was lost over the shakers. The SWACO desander, desilter and 1850 centrifuge were run periodically to help control the drill solids. The ROP at the start of the interval was over 100 meters/hour, decreasing to 40-60 meters/hour in the Alibag shale.

The Details

The cement from the previous 13?-in casing was drilled with seawater and the hole displaced to a new SILDRIL system. A hi-viscosity spacer was built by adding 1 ppb XCD polymer to mud from the active system. The displacement was good with a sharp interface seen at the shale shakers. After displacement, the system was circulated to shear the polymers. An FIT was carried out to a 12.5 ppg EMW. Despite the high penetration rates at the start of the interval (over 100 m/hour), the solids control equipment coped comfortably. The vast majority of the cuttings were removed at the scalping shakers. The hole was drilled without problems to 1,390 meters. A wiper trip was made back to shoe to check on hole condition. The trip was good with only one small tight spot being seen. Drilling continued to 1,431 meters where a pit gain was seen in the Bombay Limestone. The mud weight increased from 10.4 to 10.6 ppg. During the drilling the SILDRIL system became heavily aerated. The aeration was caused from the use of a locally manufactured lubricant, used in the mud formulation.

The lubricant hydrolyzed, forming a soap, which resulted in the foaming. The additions of the lubricant were stopped and a defoamer was added to the system to alleviate the foaming problem. With the foaming problem under control, drilling continued in the Bombay Limestone and Alibag Shale without any problems to 1,709 meters, the interval TD. Silicate measurements were monitored in and out during the drilling phase to establish the rate of depletion. Based on an average silicate concentration of 8.2% by volume, the depletion rate was established to be 0.297 bbl/meter (0.091 bbl/ft). This works out to 0.62 bbl of SILDRIL L per barrel of cuttings drilled. The potassium ion concentration in the system was monitored using the sodium perchlorate centrifuge method. The value for the KCl concentration remained stable with little evidence of depletion. The SILDRIL system performed very well and gave good clay inhibition, hole stability and drilling performance.