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Abstract

The most widely used drill-in and completion fluids for seepage loss control in payzone are water-based mud containing polymeric additives and sized calcium carbonate or sized salt. These inorganic bridging solids are very effective in sealing the pore throats of the porous matrix and preventing formation damage but owing to their brittleness they will grind down during circulation and handling. This particle degradation makes it difficult to maintain the correct particle size distribution during drilling operations and can cause a deep penetration of the finest solids into the formation.

A novel, solids-free drill-in fluid system containing a biopolymer as viscosifier and a highly crosslinked starch and microfibrous cellulose as fluid loss control agents has been developed for drilling and completing low permeability formations. This fluid composition provides the qualities of both good drilling mud and a non-damaging completion fluid. This paper presents the laboratory results obtained from coreflooding tests carried out on this solids-free drill-in fluid in limestone cores with permeability lower than 100 mD. The synergistic interaction of the biopolymer with a highly crosslinked starch and a microfibrous cellulose material promotes the quick formation of a very thin (ca. 50-60 μm) and impermeable filter cake which prevents polymer and drilled solids invasion into the core. The results show that the filter cake can be completely removed by backflooding and the permeability can be completely recovered. The non-damaging effect of this drill-in fluid was also observed when its density was increased to 1.48 g/cm³ by potassium formate brine. Good filtration and rheological properties were also maintained when simulated drilled solids, e.g. reactive and non-reactive clays were added. Core plug invasion and thickness and the morphological structure of the filter cake, investigated by means of Scanning Electron Microscopy, is also discussed.

Introduction

Recent advancement in drilling and completion technologies for maximize return on drilling investments have led to the development of totally new types of drilling and completion fluids. Advanced drilling technologies, like high angle well, multilateral or slim hole tecnologies^1-3, require fluids that minimize pressure losses while maintaining effective suspension properties and a non-damaging behavior.

Therefore there is a particular need for improved drilling fluid systems that meet both drilling and completion requirements and can be successfully applied for drilling operations in complex formations or in depleted reservoirs. However, in addition to controlling the frictional pressure, minimizing fluid invasion is always the priority in order to avoid formation damage.

In the last decade the major Service Companies developed a new class of drill-in fluids^4-6 that satisfies most of the requirements of a good drilling and completion fluid but further improvements should still be made in their formulation to ensure both maximum well productivity and minimum rig time. Most of these fluids contains biopolymers and soluble sized bridging solids but the reduction of the bridging solids concentration and the use of polymeric additives with stronger shear-thinning characteristics and higher temperature stability are current challenges. Density adjustments to the system should be made with high-density brine⁷ rather than solids to maintain an ultra-thin filter cake and a low viscosity profile.

It is known that bridging solids, such as sized calcium carbonate or sized salts, allow the quick formation of an effective low permeable filter cake if their particle size distribution has been selected according to the porosity and permeability of the reservoirs. Furthermore, if the particle size distribution of the bridging solids is specifically designed to seal the pore throats of the formation, their concentration in the drill-in fluid can be reduced to a minimal amount. However during drilling operations there are inevitable processes, such as particle degradation during circulation and handling, which make it difficult to maintain the correct particle size distribution in the drill-in fluid and can cause a deep penetration of the finest solids into the formation^8,9.