When centrifuges are properly deployed, it 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.
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 reach this goal is affected by several factors other than drilling fluid inhibition. For example, formation solids reactivity is the combined measure of the potential for a material to cause a negative impact to the drilling activities by material hydration or dispersion. Centrifuge design and operating parameters also play a role. However, the goal of achieving a dry solids discharge should not be prioritized over the goal of achieving the proper colloidal solids cut.
The application of traditional centrifuging techniques removes both the ultrafine and colloidal solids regardless of their classification as formation solids (drilled solids, low gravity solids or LGS) or drilling fluid solids (weighting agents, most commonly barite). The goal would be to remove all suspended solids above a targeted particle-size distribution. Then new drilling fluid solids would be added to the system. In barite recovery, the traditional intent is to maintain the larger solids, specifically barite and those solids classified by the American Petroleum Institute (API) as fine and ultra-fine for return to the drilling fluid.
Defining Traditional Barite Recovery
For Barite Recovery operations, service providers pair two centrifuges together, in which the first centrifuge targets solids between a 10 and 100 μm range and the second and the second purportedly removes those solids less than 10 μm. This 10 and 100 μm range is the sweet spot within the particle size distribution curve for barite. The lower end of this range (10 μm) is set by the centrifuge’s applied G-force, where the upper end of this range (100 μm) is set by the practical suspended solids cut achieved by the primary solids control system.
The influent to the barite recovery centrifuge comes as a slip-stream from the active mud system. This means that the full circulation volume of the active mud system is not sent through the centrifuge system. The first centrifuge’s solids cake is returned to the active mud system with the goal of recycling barite, while the centrate is plumbed to the second centrifuge.
Figure 1 shows the typical barite recovery process.
Colloidal Solids’ Effect on Plastic Viscosity
Many centrifuge operators do not understand that drilling fluids’ performance and the associated plastic viscosity are driven by the colloidal solids content, not low-gravity solids. Even fewer operators understand the difference between low-gravity solids and colloidal solids.
Barite has a particle size distribution that ranges from 1 to 100 microns (predominantly ultra-fine and fine solids). API specifies that barite should achieve a size distribution where the percentage of material greater than 75 µm is minimized, while ensuring that the percentage of material less than 6 µm is less than 30 percent by weight.
Bentonite particle size distribution can range from less than 1 to 10 µm (colloidal and smaller ultrafine solids). Bentonite solids have a much smaller particle size distribution than barite. This is the reason that bentonite is typically considered a thickening agent to increase viscosity, while barite is simply considered a weighting agent. Barite has only a modest impact on viscosity, because the aggregate surface area of the solids is lower than the surface area exhibited by the solids found in bentonite. This is because the negative effects of elevated plastic viscosity are predominantly driven by the available surface area exposed by colloidal solids, which are more prevalent in bentonite than barite.
Despite the fact that the particle size distribution bands are fairly well defined, measuring solids within these ranges is complicated. Many centrifuge operators mistakenly rely on a retort analysis. Though the retort is a practical tool to measure the total aggregate mud weight, it is not helpful for defining the change in plastic viscosity (such as an increase in total surface solids surface area as it directly relates to an increase in the colloidal solids content) associated by the continued and natural degradation of drilling fluid and formation solids into colloidal solids. To pinpoint the changing conditions (rheology) of the drilling fluids through the well cycle, details relative to the particle size distribution must be captured.
A particle size distribution analysis can shed light on the total surface area exhibited by the drilling fluid suspended solids. Because this test is not considered practical or cost-effective to complete on the rig site on a daily basis, most operators choose retort analysis instead.
Unfortunately, by relying exclusively on the retort analysis, centrifuge operators and mud engineers are unable to ascertain the characteristic life cycle of the colloidal solids.
Many mud engineers assume that when the mud weight increases, as defined by the retort analysis, the only logical option is to increase the treatment capacity of the centrifuges or increase the applied G-force. However, since the centrifuges treat a small percentage of the total circulating volume and centrifuges are unable to manage colloidal solids, operators are left fighting a losing battle.