Drilling fluids, also called mud, serves many critical roles in drilling and completions. These include:
- Pressure control - preventing a blowout
- Lubrication and cooling of the drill bit and drill string
- Corrosion control
- Transporting drill cuttings away from the drill bit towards the surface
- Stabilizing formations by coating the walls and providing pressure
- Sealing permeable formations, called “mud cake”
- Suspending drill cuttings when the drilling operation is paused (called circulating)
- Transmitting hydraulic horsepower to the drill bit and other down-hole tools - such as mud motors
- Drill string torque control in deviated wells
Using drilling fluid is the only way to control the pressure inside an open well and avoid fluid flow or collapse of the well. The mud weight is adjusted by adding certain chemicals to achieve a pressure balance between the pressure inside the wellbore and the pressure exerted by fluids in the surrounding formation. An effectively planned and maintained drilling-fluid system can significantly increase the penetration rate when drilling, solve a number of technical well-related issues and can protect the reservoir from damage (e.g. from the invasion of fluids and solids into productive zones).
Mud is mixed on (near) the drilling rig (although occasionally it is mixed in tanks onshore and then shipped to the rig by vessels) in mud tanks and is pumped to the well by a system of mud pumps and pits, through a hollow in the drill pipe. Once the mud reaches the drill bit, it (the mud) is sprayed out from nozzles on the drill bit. By maintaining the pressure using mud pumps, drilling fluids travel upwards within the annulus (the space between the drill string and the walls of the wellbore). As the mud flow to the surface, cuttings generated in the drilling of the well are transported to the surface. Once on the surface, the mud is cleaned and the solids are removed and then it is re-circulated back to the well, subject to mud checks and the addition of any required chemicals.
There are three types of drilling fluids, which are described below. Although water-based mud (WBM) is the most widely used system, it does not offer the same advantages as other types of fluids, such as oil-based mud (OBM) or synthetics-based mud (SBM). Collectively called non-aqueous fluids (NAF), OBM and SBM provide far more superior performance in terms of stability of the well, rate of penetration and overall drilling efficiency, but they are costly systems to apply. In some instances, OBM and SBM may be the only technical solution and WBM may not be an option due to the down-hole conditions. Over the last 10 years, drilling-fluid service providers have been able to develop better-performing WBM called high-performance water-based mud (HPWB), which in many cases provide similar benefits as OBM and SBM, yet at a lower overall cost.
Over the last 5-7 years, drilling-fluid service providers have been able to develop better-performing WBM called high-performance water-based mud (HPWB), which in many cases provides similar benefits as OBM and SBM, yet at a lower overall cost.
Water-Based Mud Systems
Water-based mud is the most basic system and is composed of water (fresh and seawater), clay (bentonite), brine and other chemicals/additives. KCl-PHPA (+ glycol) is a classical WBM type and is the most cost-effective solution with moderately water-sensitive shales. Sault PHPA (+ glycol) WBM can be used with less-water-sensitive shales and when the utilization of KCl (potassium chloride) is not allowed. Other systems include KCL polymers, milling fluids and lignosulphonate mud.
An HPWB mud system provides similar benefits as OBM and SBM, yet at a lower total cost. HPWB mud includes unique salts, special shale stabilizers, inhibitors, and viscosifiers. HPWB mud is commonly used and is beneficial where the performance characteristics of OBM or SBM are required; however, it is cost-prohibitive to use them due to the high costs of waste management and disposal, or facility limitations that restrict the installation of waste handling and processing equipment. The costs of HPWB mud can sometimes exceed the costs of OBM or SBM, but as the disposal costs of non-aqueous fluids, i.e. OBM and SBM, are high, their total cost is higher than that of HPWB mud.
Oil-based Mud (OBM) and Synthetics-Based Mud (SBM)
Oil-based mud is a drilling-fluid system composed of base oil, water, additives, and chemicals. Depending on the application and regulatory and environmental requirements, the base oil could be diesel, kerosene, low-toxic mineral oil or synthetic oils, such as esters, olefins or paraffins. Effective shale inhibition is one of the main advantages of OBM. The ability to support drilling in a high-temperature environment, where the water component of WBM would be boiled out, is another key advantage of OBM. In addition, OBM can be reused many times and can be stored for a long period of time.
Due to the toxicity of OBM, disposing drill cutting and waste fluids generated by OBM is prohibited in offshore applications. Drill cuttings and waste fluids are thus processed and shipped to shore for disposal. SBM systems were developed to address this constraint. There are many applications worldwide where the waste generated by SBM is disposed of offshore; however, it is still prohibited in the Caspian Sea and the North Sea, where all drill cutting and waste fluids generated by SBM must be shipped to shore for processing and disposal.
The major technical drawback of using OBM or SBM is, due to their hydrocarbon nature, which may show as spurious hydrocarbon indications in the reservoir where there is none and can cause the contamination of cuttings and core samples. However, complex analytical tools are available to mitigate this problem; for instance, in the GCC region, base oil such as Escaid 110, Saraline 185V, DF 1 and diesel are used.
The range of chemicals and additives used in the drilling fluids is extensive, but the most widely used are: bentonite, barite, calcium carbonate, calcium chloride caustic soda, polyanionic cellulose (PAC), xanthan gum, polysaccharide, sodium chloride, soda ash, various lost-circulation materials (LCM), glycol, mud thinner and H2S scavengers. The roles of the chemicals and additives are diverse and mainly involve providing viscosity and rheology control, shale stability, cooling and lubricating, lost circulation, inhibition and encapsulation, mud weight and corrosion control.
Completion fluid (aka work-over fluid) is a fluid used to complete the well and is normally a salty solution made up of e.g. chlorides or bromides, which is also called Brine, as must be free of any solids. Completion fluid should be compatible with the innate formation fluids and rock, as another type of fluids (e.g. drilling fluids) can damage the productive zones. In addition to cleaning the wellbore, after the drilling is finished, completion fluid is used to control the pressure down-hole, prior to and while well-completion operations are in progress.
On average, the cost of drilling fluid is 5-20% of the total well cost. However, a small marginal increase in spending on drilling fluids could be offset by the superior drilling-fluid performance achieved, thus can result in great cost savings, cost avoidance, and a lower total well cost. Drilling fluids are becoming a higher percentage of a well cost and the trend is growing.