Abstract

During the last several years, drilling deviated wells through highly bentonitic shallow formations and unconventional coal bed gas sands in the San Juan area of the Rocky Mountains has indicated the need for fluids as inhibitive as a 7% KCl/polymer system. Operators have experienced advantages with this type of system in terms of wellbore stability, penetration rates, wellbore workability and well productivity. However, increasing environmental restraints in the area make the economics of this system less desirable in terms of haul-off and disposal costs. Core samples of reactive shales in the area were analyzed to determine if a freshwater-based system could be formulated to achieve similar performance to what was previously achieved with the 7% KCl system. Several different freshwater-based muds were tested in the lab, including some high performance water-based fluids. This paper describes criteria that enables a fluid to be called a high performance fluid and reviews the data that yielded a "fit-for-purpose" high performance fluid designed for the specific clay mineralogy of the San Juan area. A field trial was initiated to decide if the lab results could be verified by actual drilling data. Performance data from the trial wells are reviewed and compared directly against the previous successful system. The paper illustrates how a fit-for-purpose high performance water-base mud can help the economics of unconventional gas sand drilling.

Criteria for High Performance Water-Based Fluids

High performance water-based fluid (HPWBF) is a term that most of us in the industry have come to understand within the bounds of our personal experience and imagination. Until recently very little has been published regarding criteria that clearly distinguishes "high performance" water-based fluids from conventional water-based systems. Environmentally-friendly inhibitive water-based fluids that provide borehole stability, high rates of penetration (ROP), extended bit life and trouble-free trip time is generally what comes to mind because those are the most noteworthy standards set by invert emulsion systems. However, these are the expectations we hold for a "high performance" fluid and not the criteria needed to guide the building principles for such a fluid. In most drilling programs the drilling fluid is formulated as "fit-for-purpose" and the individual additives are chosen based on their economical contribution toward specific ends.

To build a freshwater fluid with a foundation necessary to achieve the performance results expected by the operator in the San Juan field, certain rigorous design standards had to be met. This project adopted the criteria for HPWBF developed by a major drilling fluids service company. The criteria are listed below:

  • Highly inhibitive: Fluids that are highly inhibitive reduce the amount of swelling and dispersion experienced by reactive formation clays when exposed to the fluid during drilling operations. With the correct inhibitors, the borehole remains closer to the bit size, which enhances hole cleaning. Additionally, inhibitive fluids allow the borehole to remain stable for long periods of time and trips are less troublesome. Highly inhibitive fluids also reduce the dispersion of fine solids within the mud system, resulting in a higher efficiency of mechanical removal.

  • Low colloidal content: Based on studies done in the early 1970s1, 4, ultra-fine colloidal solids have the most detrimental effect on ROP. To promote higher penetration rates, high performance water-based fluids should be formulated with minimal colloidal material and have the inherent ability to reduce the build-up of ultra-fines while drilling.

  • Shear-thinning: Numerous studies indicate that fluids that demonstrate a viscosity increase at reduced shear rates are more effective at cleaning for a given hole inclination and pump rate.2 Adequate hole cleaning is important in providing a trouble-free borehole; therefore, all high performance water-based fluids should be designed to employ the advantages of shear-thinning. However, that is not always the case.

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