Hybrid system combines benefits of WBMs, EBMs

New developments in drilling fluid technology have increased the performance of water-based muds (WBMs), which traditionally have an acceptable environmental profile, as well as the use of new synthetic-based oils for emulsion-based muds (EBMs) that are significantly less toxic than their diesel-based counterparts. Despite these less toxic options, the use of diesel-based EBMs is still widespread and represents the fluid used in the overwhelming majority of wells in many areas. Operators oftentimes underestimate the importance of selecting the right fluid for the job or may not even consider all of the options now available.

Operators also often underestimate the importance of drilling fluids in designing a successful well. The drilling fluid can make up 15% of the total projected budget, thereby representing a significant portion of the costs associated with drilling. The drilling fluid also is the only component that comes into contact with every major aspect of the operation. It even comes into contact with rig hands and therefore can represent a significant health risk if toxic or hazardous chemicals are used. It also can influence ROP and bit life, affect casing and drillpipe corrosion and longevity, and impact formation stability and permeability. Formation stability accounts for a large portion of delays on drilling operations in shale formations, and the condition of the drilling fluid plays a major part in mitigating any potential problems.

The benefits of EBMs include superior ROP, shale inhibition, and rheological stability (Figure 1). However, the degree of these benefits may be oversimplified. For example, there are fluids that can deliver higher ROPs such as brines or foam/air drilling. In addition, although EBMs are excellent at preventing the hydration of shale, there is some research suggesting that they can actually cause wellbore instability by increasing rates of dispersion in some cases.

FIGURE 1. This figure compares the benefits and drawbacks of drilling fluid systems. (Images courtesy of ViChem Specialty Products LLC)

There are additional disadvantages as well. One major disadvantage is the potential for EBMs to damage the pay zone through emulsion block. Another major drawback in the use of EBMs is the reduced ability to control losses, which can quickly cost the operator millions of dollars and extend emulsion block beyond the near wellbore. Because most lost circulation material is water-wet, it does not function efficiently when used in oil-wet fluids. The water-wet nature of most formations also contributes to seepage of EBMs as well as the likelihood of significant losses.

WBMs

WBMs address many of the HSE concerns associated with EBMs but are generally considered to have difficulty providing the level of performance in terms of shale inhibition and ROP necessary to complete the technically demanding wells that are common today (Figure 2). Some of the performance issues have been addressed with the development of high-performance WBMs (HPWBMs). HPWBMs generally use polymers for viscosity and filtrate control. Polymer-based drilling fluids are designed to be more resistant to contamination than clay-based systems and allow the use of a wider variety and increased concentrations of lubricants and shale inhibitors. Despite all of this, they have not fully replaced EBMs due to a lack of confidence in their ability to perform.

There are other significant advantages to the use of WBMs as well. These advantages include the low unit cost and flexibility in formulation. Without the cost of base oil and large volumes of expensive emulsifiers, it is possible to build most WBMs at a fraction of the cost of EBMs. There

also is an almost infinite number of formulations available depending on the challenges expected. The increased ability of WBMs over EBMs to control and prevent losses is another major advantage. Lost circulation can add significantly to the total cost of fluids for the well and be a serious safety hazard. Finally, WBMs can be built and disposed of on site with products that are readily available. This dramatically reduces over-the-road liability and truck traffic at the rig site, which is an often overlooked benefit of WBMs. Truck traffic is one of the largest complaints by the public against oil and gas operations, and the value of reducing local impacts from operations cannot be understated.

FIGURE 2. The average cost and performance characteristics of multiple wells drilled in the Eagle Ford using WBMs, EBMs, and the MHA system are compared. Error bars represent the maximum and minimum values.

Hybrid system

Unlike many other aspects of oil and gas operations, there has been little advancement in fluid-related technologies in recent decades. Synthetic-based EBMs still do not meet regulatory standards for whole mud land farming or discharge into the sea, and HPWBMs have not met the performance standards of EBMs. This absence in technology was noted by the founders of ViChem Specialty Products LLC, who set out to fill the need for an environmentally friendly fluid with performance comparable to that of EBMs.

ViChem’s Multi-Hydroxyl Alcohol (MHA) drilling fluid system was developed in collaboration with scientists and field engineers and was brought to market in early 2011. The idea was to combine the performance that a hydrocarbon-based fluid provides in EBMs with the full range of benefits that WBMs realize due to their water solubility. The MHA system is a hybrid between WBMs and EBMs. The base fluid is a mixture of low polar organic compounds that have lubricity and inhibition similar to oil because of their hydrocarbon influence. However, unlike petroleum products, the MHA molecules contain hydroxyl groups on the carbon chain, allowing the fluid to be soluble in water without emulsifiers and completely nontoxic to the environment.

Once laboratory studies suggested that this hybrid technology was feasible, ViChem scientists teamed up with field engineers to design a program for field trials. Horizontal wells had been drilled into the Woodbine sands for many years prior to 2011. As drilling operations expanded in the Woodbine field, the sand target became very narrow, making it nearly impossible to stay in zone and out of troublesome shales. The Woodbine sand is located between layers of Eagle Ford shale, which is notorious for its reactivity and sensitivity to hydration. Common deviations from the target sand exposed sensitive shales, requiring a level of inhibition unattainable with current WBM options.

The first MHA well was drilled in December 2011 as an offset to a previous well that had difficulties attributed to the shortcomings associated with the use of WBMs. The project using the hybrid MHA system was completed without incident, reduced drilling time by almost 40%, and increased the length of the lateral by almost 610 m (2,000 ft). It is important to note that this field trial was on offset wells using the same well design, yet the MHA system was able to reduce the number of drill days and increase the operator’s access to the pay zone without switching to an EBM.

FIGURE 3. Results of environmental and toxicity testing of MHA whole mud, components, and cuttings collected on a well using the MHA system enabled the cuttings to be disposed of at a local landfill.

In late 2011 ViChem had the opportunity to run a project drilling two offset wells from the same pad in the Marcellus shale play in Marshall County, W.Va., to compare the performance of the MHA system to that of a synthetic-based EBM. The MHA system matched the ROP of the EBM and actually completed the project in one less day because of logistical delays experienced with the EBM. The total fluids and disposal cost of the MHA well was approximately half the price of the previous well drilled with the EBM. After completion of this project, toxicity testing of whole cuttings was completed to obtain regulatory permission for disposal at a local municipality. The cuttings passed all of the required tests (Figure 3), and the operator was granted approval, consequently saving an estimated US $100,000 in logistical costs and thousands of transportation miles on each well. In this case, the use of the new system allowed the operator to reach its drilling objective at half the cost of an EBM, reduce the potential for an environmental incident by using a nontoxic drilling fluid, and increase community support by significantly reducing truck traffic.

The MHA system has continued to expand in usage in the Eagle Ford and Marcellus formations and has now expanded to the Pearsall and Permian basin shale plays.

The oil field has an obligation to better communicate to the public its ability to responsibly develop energy sources. In this manner, it is possible to play a more significant role in guiding upcoming regulations. These considerations were among the major objectives behind the development of the MHA system. The social and environmental drivers have turned out to be as important as the performance characteristics and economic advantages of the fluid in influencing its success.