The overall efficiency of cuttings removal by the solids-control system, Es, can be expressed as:
Es = E1 f 1 +( 1 – E1 )E2 f 2 + ( 1 – E2 )E3 f 3 +( 1 – E3 )E4 f 4 (1)
where E1–E4 are solids-removal efficiencies (by volume) of the shale shaker, desander, desilter, and centrifuge, respectively, and f1–f4 are volume fractions of mud processed by these separators. The equation has little practical use because the efficiencies E1–E4 are dependent upon separators’ inputs, which in turn depend on the variable content of the flowline mud. However, Eq. (1) is useful for the design of a new system configuration, and also for the evaluation of solids-control separators at work. In the latter case, the efficiency of each separator should be determined using API procedures; then the overall efficiency should be calculated from Eq.
There are a few direct methods available at the well site to determine the overall efficiency of cuttings removal. The methods are based either on density measurements or water dilution records. Calculation of the overall separation efficiency using mud density measurements at the suction pit usually takes a long time (a day) and requires several cycles of mud circulation. The other method, measurement of the density difference between flowline mud and suction pit mud, does not give enough accuracy with the use of a mud balance. Alternatively, determination of reactive cuttings in the mud using the retort and the Methylene Blue tests does not have the precision required to detect the increase of clay concentration before it affects the mud rheology. An interesting method has been presented to determine a solids-control index (SCI) from the monitored water dilutions required to control drilled solids. Although very practical, the method requires monitored water usage for dilutions and cannot be used for weighted mud systems.
Several attempts have been made to develop a mathematical computerized
model of cuttings removal. All of these attempts use the steady-state material balance approach with known and constant values of separation efficiencies of system components. They do not consider the relationship between the separation efficiency and particle size distribution, solids throughput and liquidphase properties of the processed mud stream. Also, practical verification of the models is limited because no solids-control instrumentation is available on drilling rigs. More successful efforts have been made to develop experimental models of single separators: hydrocyclones (mud cleaner,desander and desitler), shale shakers, and decanter centrifuges, together with the analytical and field-deployable techniques for evaluation of the separators’ performances.
Emphasizing the efficiency of solids-removal may lead to the generation of excessive volumes of drilling waste. For any separator, whether shale shaker, hydrocyclone (mud cleaner, desander and desilter) or centrifuge, a strong correlation exists between solids separation efficiency and volume removal of the associated mud liquid phase. Hydrocyclones (mud cleaner, desander and desilter), for example, when operated at 0.6 solids separation efficiency, may remove up to nine times more liquids than solids, as shown in Fig.1. Generally, any increase in Es would result in increasing values for liquid removal, represented by the liquid removal ratio, R (the ratio of the volume of removed liquid to the volume of removed solids). The correlation between E and R is unique for solids-control equipment and drilling mud used in the well. Theoretical calculations indicate that maximizing the efficiency of solids separation may result in up to a 50%increase of drilling waste volume. Hence, there is an optimum value of Es that gives a minimum volume of waste.
In the late 1980s and over the 1990s, a considerable improvement was made in solids-control separators. One significant improvement was in shale shakers and screens. Drilling rigs are now equipped with two or more linear motion shale shakers. Some rigs may have as many as ten shakers, several of which are used as scalping shakers upstream of the fine-screen linear-motion shakers. The linear-motion shakers are often fitted with screens having an equivalent mesh size of 150 or more, which results in the removal of fine particles. The dramatic reduction in the size of the particles that can be screened from the drilling fluid has led to improved drilling-fluid performance and to a reduction in the volume of fluid required for drilling a well and discharged at the end of drilling the well. In addition to shale shakers and screens, the importance of the entire mechanical solids-removal system in reducing waste volumes from drilling operations has become better understood, which has resulted in the development of closed-loop drilling systems.
Table 1 Development and performance of closed-loop drilling systems
|Surface hole removal efficiency (%)||15||46||68||81|
|Production hole removal efficiency (%)||20||67||80||89|
|Surface hole mud and disposal costs ($)||10,200||7,800||6,000||45|
|Production hole mud and disposal costs ($)||25,600||14,300||8,300||4,800|
|Total costs ($)||35,800||22,100||14,600||9,300|
The closed-loop system approach requires that the drilling waste should be disposed of at the drilling site and not taken out of the loop for offsite disposal. From the standpoint of ECT methodology, closed-loop system technology integrates on-site disposal techniques with the drilling process (the environmental boundary is drawn around the drillsite, reserve pits and land treatment area). The drilling mud loop is partially closed through improved efficiency of the solidscontrol separators. The loop is finally closed through ultimate disposal on-site within the process boundaries. Table 1 shows the improvement in cuttings separation (hole removal) efficiency and economics resulting from the closed-loop system approach. Closed-loop technology employs high-quality solids-control separators in various configurations. Sometimes these systems are provided as skidmounted tandems known as unitized solids-control systems. Two types of unitized systems are available: one built by the solids-control equipment vendors and the other custom designed and built by operators.