When centrifuges are properly deployed, they can have dramatic effects on drilling performance. In fact, without them, many of the advances made in drilling would not have been considered achievable. Classically, we believe that centrifuges can achieve two common goals:
- Reduce the content of drilled fines, specifically colloidal solids, and
- Perform barite recovery in order to reduce drilling fluid additive costs.
Respectfully, each goal presents a valiant initiative. However, when combined they also present a number of complications that are typically overlooked by most modern solids control service providers.
In understanding these complications, it is important to recognize that the ultimate goal of a solids control centrifuge is to reduce the plastic viscosity of the drilling fluid. This improves the drilling rates of penetration and reduces the damaging effects of accelerated wear on bits, mud pumps, and related equipment. Further, if all goes as planned, by controlling the colloidal solids, significantly less mud make-up dilution is required. In essence, when properly used, centrifuges should enhance the drilling fluid properties, thereby improving rig performance (i.e. increased rates of penetration, improved cake wall stability, reduced bit torque and reduce pipe drag). Concurrent to these benefits, centrifuges also can be used to lower waste disposal costs by reducing the volume of waste drilling fluid and reduce raw material additive costs by maintaining the target properties of the drilling fluid. Generally speaking, this is the canned sales pitch offered to drill rig operators, used by solids control service companies, in order to market barite recovery and solids control services.
Despite these goals, the evolution of the drilling industry over the last few years has resulted in the rapid deployment of centrifuges that are unable to achieve the results targeted or marketed. Though the centrifuges available in the market have indeed improved, modern drilling practices have evolved greatly in the last decade. Modern drilling rigs are continuing to set new standards by drilling deeper, faster, and longer (relative to the pervasive utilization of directional drilling techniques). Given the recent evolution of drilling, there are indeed concerns growing as to whether or not the application of centrifuges has evolved at an equal pace. In fact, it was almost 10 years ago when concern was initially raised by Eugene E. Bouse in his May 4th, 2013 E&P Magazine article entitled “The Use and Misuse of Centrifuges”. As noted by Mr. Bouse, “Unfortunately, misuse has become more common, and many of the costly present-day practices are so ill-conceived that they are actually detrimental to both mud quality and waste minimization efforts.” As will be carefully consider through the remainder of this document, Mr. Bouse was pointing to several concerns relative to barite recovery services.
Section 1.0 - Defining the Basic Centrifuge Solids Control Terms
Centrifuges are deployed in order to “cut” solids from the liquid stream; essentially creating two separate streams from the incoming influent. We typically consider the cut to be the “underflow” (a.k.a. “solids discharge”, “cake”, and/or “heavy phase effluent”) and the cleaned liquid stream (a.k.a. “centrate” or “light phase effluent”) to be the “overflow”. The centrate will contain most of the liquid and the finer solids. The cake will contain less liquid and the coarser solids. The goal is to have the cake as dry as possible with the lowest volume of surface wetting liquid achievable. The ability to achieve this goal is a direct function of the drilling fluid inhibition, formation solids reactivity (defined as the combined measure of the potential for a material to cause a negative impact to the drilling activities by material hydration and/or dispersion), centrifuge design parameters (i.e. conical section length, conical section inner diameter and beach configuration), and centrifuge operating parameters (i.e. pond depth, g-force applied and conveyor speed). However, it is important to note that the goal of achieving a “dry” solids discharge should not precede the goal of achieving the proper colloidal solids cut. Table 1 highlights the physical effect each of these parameters has on the ability of the centrifuge to make a cut.
The application of “Traditional Centrifuging” techniques removes both the ultrafine and colloidal solids regardless of their classification as formation solids (a.k.a. drilled solids, low gravity solids or LGS), or drilling fluid solids (a.k.a. weighting agents – most commonly barite). The goal would be to remove all suspended solids above a targeted particle size distribution, whereby new drilling fluid solids would be added to the system. Relative to Barite Recovery, the traditional intent is to maintain the larger solids, specifically barite and those solids classified by API as fine and ultra-fine (assuming that most medium, intermediate and coarse solids were appropriately removed by the primary solids control systems; the flow line shakers) for return to the drilling fluid.