Summary

Horizontal wells are commonly completed without casing and the fluids used to drill said sections can have a significant effect on the production outcome. Mixed metal hydroxide (MMH)/bentonite-based fluids are compared to two commonly used reservoir drill-in fluids (DIFs) with respect to filter-cake characteristics and regain in permeability. The MMH fluids are found to produce filter cakes with unique features which result in efficient backflow without need for chemical treatment. Return permeability results which equal or exceed those for more commonly used DIFs raise questions over the conventional view that bentonite-containing DIFs are harmful to reservoirs.

Introduction

It has long been recognized that fluids used in the process of penetrating hydrocarbon-bearing reservoirs can impair the producibility of said reservoirs. In an era of depressed oil prices, the industry has been forced to investigate routes to more cost-effective production of hydrocarbons; this has been the primary driving force for the development of and growth in horizontal drilling of reservoirs. Interest in minimizing the drilling fluid's impact on the producibility of the reservoir has reached new heights and has spawned countless conferences, industry studies, and technical publications. Most major service companies have developed "drill-in" fluids (DIFs) designed to facilitate the penetration of reservoirs while allowing for effective cleanup and efficient flowback. (Our focus here is on open-hole, nonperforated completions.)

There is broad agreement within the industry as to the most common sources of damage, and the majority of publications on the subject detail broadly similar damage mechanisms. While there appears to be a consensus on the principles involved in minimizing the impact of the wellbore fluid on the reservoir, there is a growing divergence in the approaches adopted in the application of these principles.

Jiao and Sharma1 investigated the role of filter-cake formation in the minimization of particulate invasion, and concluded that the early time period of exposure is critical. Rapid formation of a strong, low-permeability cake is central to minimizing fines invasion; it is this consideration which has led to the common practice of adding bridging materials to aid in rapid cake development.

In a more recently published discussion on formation damage issues,2 Ali gave a concise statement of the expectations of the filter cake in these circumstances, a view which was broadly echoed by other participants. He stated that not only was the fluid required to provide adequate protection through cake formation, but also that the cake must be easily removable. The means by which the latter operation is most efficiently achieved provides an area for investigation. The two philosophies most commonly embraced were both addressed in the discussion. Ali referenced the need to remove the cake using breakers while McLeod preferred "cake lift-off" during the well flow (a route which has been investigated by Browne and Smith3). In the same discussion, Peden noted that "prevention is better than cure," and it was on this premise that the project design described herein was based.

This work was performed on the basis that a preferred route to drill-in fluid design embodies the ability to rapidly develop an external cake (as a means of limiting early time fines invasion), which cake should be easily lifted off simply through flowback. This approach is preferred to that in which breakers and soaks are used to attack the cake, as the overpressure circumstances of such operations are likely to cause movement of the fines into the pore structure, thereby reversing the protection originally afforded by the cake. Browne et al.4 identified a number of areas of concern with respect to the use of breakers. These encompass logistical considerations and others which directly impact the matrix of the reservoir itself.

Scope of Work

In the studies detailed hereafter, open-hole, nonperforated completions and that component of reservoir damage emanating from the invasion of fine (mainly drill) solids were addressed specifically. The project focused on various performance aspects of three commercially available drill-in fluid types, namely, polymer/sized salt, polymer/sized carbonate, and bentonite/mixed metal hydroxide (MMH)/sized carbonate. Other fluids were included at various stages for reference purposes. Three natural test substrates, covering a wide range of permeabilities, were used.

Initial work compared filtration characteristics of simple laboratory-prepared weighted and unweighted fluids. Filtration rates were studied and scanning electron microscopy (SEM) imaging was used to study the fate of the fluid-derived solids during fluid/rock interactions. All subsequent work was conducted using weighted fluids, and extensive use was made of SEM imaging throughout. Return permeability and cake lift-off studies were performed, again with laboratory-prepared fluids.

The final area of experimental investigation addressed the behavior of a field bentonite/MMH/carbonate fluid in light of the insight gained from the study of laboratory-prepared fluids of the same type.

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