Chrome-Free WBM A Viable Option For Ultra-HP/HT Applications

Typically, oil-base drilling fluids are preferred over their water-base counterparts in applications where bottom hole temperatures (BHT) can approach 260° C (500° F) with shut-in pressures to 25,000 psi, thereby requiring densities as high as 19 lb/gal (2.2 sg). However, more areas are prohibiting the discharge of oil-base fluids and cuttings, which increases waste management costs and potential liabilities. In some areas the use of oil- and diesel-base drilling fluids is disallowed altogether.

Alternatively, a newly designed, chrome-free high density water-base drilling fluid, comprising a combination of high-grade clay, novel dispersant, and specially formulated high-molecular weight synthetic polymers, shows promise as a viable environmental, technical, and economic option for extreme HP/HT applications. A field trial in an HP/HT exploration well in tightly regulated Hungary validated the performance and environmental advantages of the new aqueous-base fluid. In this well, which targeted a tight gas reservoir, static BHT was expected to exceed 170° C (356° F) with shut-in pressure greater than 10,000 psi.

HP/HT Mud Challenges

The industry has long sought a water-base mud (WBM) that would mimic, or at least approach, the higher performance of an oil-base fluid in HP/HT applications. Dispersed water-base drilling fluids, typically formulated with water, bentonite, and assorted additives to control fluid loss and provide rheological stability, are the most economical and favored aqueous systems. Advancing technology has made these fluids attractive options for more challenging applications, such as those requiring the highest level of engineering control and wellbore stability. However, designing dispersed WBM for ultra HP/HT applications has been a formidable exercise.

For example, increasing downhole temperatures tend to thermally degrade the polymers used to control fluid loss and maintain rheological stability. Under high temperatures, it becomes extremely difficult to control the gelation and the subsequent flocculation of water-base fluids containing clays and/or drill solids. The issues surrounding both thermal degradation and progressively high gel strengths are magnified with the high fluid densities required to control pore pressure when drilling ultra HP/HT wells.

With gelation being an overriding issue with aqueous fluids in elevated temperatures, chromium-containing thinners and fluid-loss additives have been used to improve rheological stability and fluid-loss properties. Chrome lignosulfonates or chrome lignites were preferred to enhance adsorption onto the clay edges, thus preventing clay platelets from bonding together. However, a growing number of areas prohibit the inclusion of toxic, multivalent metals like chrome in drilling fluids, thus spurring research into the development of chrome-free alternatives.

The data in this rheological profile were recorded during field trial in a southeastern Hungary HP/HT exploration well.

A number of additives, including lignosulfonates, mixed with more environmentally acceptable metal ions have been investigated with varying results. While both mixed titanium/zirconium lignosulfonate salt and zirconium cit- rate were shown to be reasonably effective in controlling high-temperature gelation at temperatures to 204° C (400° F), neither proved as effective as chrome lignosulfonate at higher temperatures.

Chemically, synthetic low molecular-weight anionic polymers were shown to function similarly to lignosulfonates in effectively neutralizing the positive charges on the clay edges to prevent flocculation. Subsequent work examined synthetic polymers, copolymers, and terpolymers for use as stable rheological and fluid-loss control additives at higher temperatures.

From a fluid-loss perspective, synthetic polymers, including the high-molecular weight vinyl sulfonate copolymers used in geothermal wells in the mid-1980s have demonstrated the most promise. However, a more effective alternative was needed that would likewise resist contaminants, while at the same time control rheology, and remain stable at extremely high temperatures without promoting gelation or progressive gel structure. Consequently, a research program was initiated to develop a fluid system that would equal the performance of a chrome-containing HP/HT water-base mud.

Testing protocol, results

Extensive development and laboratory evaluation of both fluid-loss control polymers and thinners/deflocculants was conducted on a 15.0 lb/gal (1.8 sg) drilling fluid dynamically aged at 250° C (482° F).

The general specifications defined the performance of the ideal fluid as one having a plastic viscosity (PV) of less than 30 cP, a 6 rpm reading between 7 and 10, an HP/HT fluid-loss filtrate of less than 20 mL at 149° C (300° F), and 500 psi differential pressure on hardened paper. The rheological properties would be measured at 66° C (150° F).

At the onset, a benchmark formulation employing chrome lignite was prepared to compare the effectiveness of the subsequent chrome-free deflocculants developed. Both the mixing and addition conditions were established and controlled to optimize performance and ensure a sound comparison between the various formulations.

Once the candidate chrome-free thinner combinations and concentrations were identified and formulated, the additives were tested with both high-grade bentonite clay and sepiolite to better determine their efficiency in a more hostile simulated environment. A stress study was completed on the fluids by increasing the weight of the formulation from 15 to 17.0 lb/gal (1.8 to 2.0 sg) with only the addition of barite. A supplementary test evaluated performance with the addition of synthetic seawater.

Of the six alternative deflocculants tested, two systems employing a mixed metal lignosulfonate and a divalent metal lignosulfonate approached the baseline rheological profile, including gel strengths, obtained with the chrome-lignite.

Fluids were then formulated and tested with the two most promising chrome-free lignosulfonate additive combinations, using bentonite and stressing the formulations with the addition of 11.5 vol% of synthetic seawater and an additional 140 lb/bbl of weighting material. In this more technically demanding lab environment, the divalent metal lignosulfonate, low molecular-weight and ionic polymer combination that was developed exhibited exceptional performance, maintaining a stable rheological profile and HP/HT fluid-loss control after aging.

On the basis of these results and earlier work, researchers were able to select optimal products for the formulation of a chrome-free WBM with the best potential to perform in the same manner as a chrome-containing HP/HT water-base fluid.

Hungary field trial

The first field application of the new WBM was in the 442 m (1,450 ft), 8/ 8 in. and 408 m (1,339 ft), 5/ 8 in. intervals of a 3,680 m (12,073 ft) exploration well in southeastern Hungary. Hungary, like much of central Europe, has strong regulations controlling the use and discharge of drilling fluids employing oil, diesel, or additives containing chrome and other heavy metals.

For this field test, the HP/HT WBM was required to maintain stable properties when weighted up initially to 16.7 lb/gal (2.0 sg) at a temperature of 204° C (400° F). Furthermore, specifications required the system to be capable of increasing density using only additional weighting material up to 19.2 lb/gal (2.3 sg) with no significant effects on rheology.

A series of laboratory tests were first undertaken to fine-tune the selected formulation to meet the predetermined performance characteristics. The two tables detail the optimized formulation as well as the test results, which provided laboratory justification for the eventual fluid to be used in the trial.

The converted HP/HT fluid was mixed easily at the wellsite and displaced in the 8 in. interval at 2,730 m (8,957 ft). The high-density fluid incorporated select high-temperature polymers and stabilizers to control fluid loss and improve cuttings suspension, hole cleaning, and filter cake quality. Owing to expected CO contamination, continuous additions of caustic soda, lime, and gypsum maintained total hardness at 300-400 mg/l, while pH ranged between 10.5 and 11.5. Centrifuges and flocculation were used to control the drilled solids content.

With mud density rising as high as 19.0 lb/gal, (2.28 sg) drilling progressed trouble-free to 3,130 m (10,269 ft) where casing was set, and the well drilled to TD with a final mud weight of 17.5 lb/gal (2.1 sg). A proprietary engineering software package was used to predict the equivalent circulating densities (ECD) for both drilling and the surge/swab pressures while tripping. The level of reactive and drilled solids was minimized to prevent high viscosity.

The well was drilled with no fluid-related issues and complied with all environmental regulations. Post-well data validated the capability of the HP/HT water-base fluid to affect tight control over properties to maintain a stable rheological profile. The ability to easily control HP/HT fluid loss actually resulted in final rheology lower than the lab test results.

This article was adapted from AADE Paper No. 10-DF-HO-37, "Environmentally Responsible Water-based Drilling Fluid for HPHT Applications," presented at the 2010 AADE Fluids Conference and Exhibition.