Water Well Journal

August 2016

Water Well Journal

Issue link: http://read.dmtmag.com/i/705618

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Page 37 of 71

I think we need to discuss in this installment of our column what I call the drilling fluid basics, for want of a better term. Sure, we risk being a bit redundant and you may have heard much of this before, but bear with me. As I often say, "You cannot discuss drilling fluids for very long before there is some repetition—the topics are all intertwined." Planning The first step as discussed in my last column (WWJ, May 2016) is proper planning. Included in the planning phase is obtaining an appropriate drilling fluid program—as property owners, hydrologists, or engineers often require a drilling fluid program. Your chosen drilling fluid vendor can either provide a drilling fluid program for you or facilitate getting one pre- pared for you. Make sure you provide them with as much data as you can so the program is appropriate for your project. Water We need to locate and verify a quality water source. The quality of the water is critical and will determine the effective- ness of any drilling fluid additives you will use. The water should be low in salt (less than 500 ppm chlo- rides), low in hardness (calcium less than 100 ppm; average hardness is around 200 ppm), and low in chlorine (minimal detectable odor). The pH and the hardness can be adjusted using soda ash (about 1/2 to 2 pounds per 100 gallons). Excessive chlorine is harder to deal with and may require a different water source or adjusting the drilling fluid additives to obtain the desired drilling fluid properties. Water quality properties for drilling fluid applications are measured using pH strips, calcium strips, or various titration methods to obtain more specific properties like alkalinities or the ratio between calcium and magnesium hardness as well as more specific chloride content. Viscosity Viscosity depends on the drilling discipline. There are two ways to clean a borehole: velocity and viscosity. Velocity is speed or rate of flow. Viscosity is the thickness of the fluid (defined as resistance to flow). When drilling with air, the fluid (air) is made thicker by adding a foaming agent. A water-based fluid is made thicker by using bentonite or a polymer. When adjusting viscosity, always use the most cost-effective product available and designed for use in a drilling fluid. The thicker the fluid, the greater the pressure required to keep it moving, requiring adjustment in other parameters. We will discuss this in more depth in a future column. Viscosity is usually measured in the field using a Marsh funnel and a viscosity cup. This measurement is a basic and a relative measurement. More scientific and specific measure- ments can be obtained using a device called a rheometer. The measurements from the rheometer can be used to determine the size, shape, and interaction of the particles in the solution. These numbers can be used to determine more specific flow properties. Filter Cake and Water Loss These two properties are closely interrelated. Any additive making the filter cake thinner and tighter will usually reduce the water loss. We want to keep the filter cake thin, tight, and relatively impermeable—usually about 2/32 of an inch. This will keep the liquid phase of the drilling fluid (usually water) from going into the formation and causing any water-sensitive materials to become wet and swell, potentially causing bore- hole stability problems. The desirable water loss is typically 15 cubic centimeters or less, depending on the nature of the formation. Filter cake and water loss are controlled by the use of a high quality bentonite in combination with proper amounts of specific polymers. Always use the most cost-effective prod- ucts available. Filter cake and water loss are determined using an American Petroleum Institute (API)-designed filter press. Weight or Density This is a measure of how heavy the drilling fluid is. Water typically has a weight of 8.33 pounds per gallon (ppg), while drilling fluid usually weighs less than 8.5 ppg when all the desired additives are mixed in. The only reason the initial drilling fluid will be higher than 8.5 ppg is because material (usually barite) has been intention- ally added to make it heavier or denser to control a subsurface pressure, an artesian flow, or an unstable formation. RONALD B. PETERSON WATER WELLS AND COMMON CENT$ DRILLING FLUID BASICS You may have heard them before, but they're critical to a job. You should verify any drilling fluid additives you choose to use are designed and fit for your project. waterwelljournal.com 36 August 2016 WWJ

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