How “Clean” is your Reservoir Drill-In Fluid?

The phrases “non-damaging” and “acid soluble material” are often used when talking about brine-based drilling fluids containing a mixture of xanthan viscosifier, starch fluid loss agent, and calcium carbonate bridging material. The inference is that any one or combination of these materials in a filter cake will protect the reservoir from formation damage, both while drilling, and when using an acid to clean up during an open hole completion. However, a different reality was discovered when first cleaning up these filter cakes using strong HCl and oxidisers. High Well Skin factors and low oil production resulted from what should have been the restoration of original formation permeability from filter cake removal. Laboratory work discovered that unless both the starch and xanthan components were completely removed prior to acid application, residual polymer material could cause damage under an overbalance well condition. This damage required stimulation (including fracking) to restore well performance. Acid also caused “worm-holing” through high permeability streaks, leaving filter cake intact across portions of the wellbore. As sufficient filter cake was not removed, the residual cake restricted well flow by exerting a high pressure drop across the low permeability formation face.

Filter cake removal technology has evolved in recent years to overcome the problems first experienced with simple acid/oxidiser combinations. It is now at a point where coverage of long horizontal sections with low reactivity breaker solutions can safely remove filter cake components over suitably long soak periods. These techniques still rely on using delayed acid solutions to attack the carbonate after initially destroying the polymers with enzymes. Success comes by carefully matching the separate polymer chemistries to specific breaker enzymes and only allowing the pre-cursors to generate acid after the enzyme’s work is complete. This avoids low pH environments which can denature the enzymes and leave polymer fragments to inhibit cake penetration or damage the formation.

A clean-up concept previously used in high permeability gravel pack completions has been overlooked until recently, namely a “Zero Well Skin” achieved by flow removal of filter cakes during production phases. This relies upon ensuring that the cake permeability is higher than the formation face permeability. Filter cake removal is not required in high permeability sandstones to optimise produced flow once the cake permeability is low enough to initiate the flow of hydrocarbons. Lower permeabilities may require destroying the polymer “glue” that holds the cake together or by partial cake removal. This is achieved with the use of enzyme and buffered organic acid soaks. Similar techniques were used to stimulate oil wells damaged by starch polymers released during 15% HCl filter cake “clean-ups” that were commonly applied in the early years of Drill-In Fluid (DIF) use. Complete destruction of all the polymer cake components is difficult when xanthan is combined with starch in most DIFs. The two different enzyme types required for a polymer break have different temperature and pH ranges of activity and can interfere on polymer breakage sites. Quite often operators minimise the xanthan component and focus on breaking just the starch. What makes a Pure-Bore® / Protect® DIF system so unique is that two potentially competing enzyme types are not required, only starch hydrolysing amylases for the polysaccharide make up of Pure-Bore® and Protect®. Amylase soaks significantly increase the filter cake permeability of Pure-Bore® / Protect® based fluids, compared to filter cakes  produced from conventional Drill-In Fluids. . This allows lower draw downs on formation pressure to result in higher flow rates than can be achieved with mixed polymer filter cakes. The same principle and technique can be applied to geothermal reservoirs producing saline brines instead of hydrocarbons. Since early 2017, Pure-Bore® as an open hole drilling fluid has been applied both in geothermal reservoirs and producing oil wells. As the concept of enhancing resource recovery through optimization of filter cake permeability gains acceptance, more widespread use of starch based DIFs is expected.