An example of using the ESD to compare different solids control options and to formulate a well plan was done for the surface hole on a well drilled in the Anschutz Ranch East Field. Data used to formulate the plan was collected while on the surface hole of an offset well that was drilled using the proposed drilling rig. A diagram of the system that was used to drill the surface hole. Less than optimum solids control practices on this well resulted in an average mud weight of between 9.2 and 9.3ppg. This was mainly accomplished by dumping over 6000bbls of mud over an 18 day period. In this area clean up costs range from 3 to 6 dollars per barrel depending on the nature of the contaminants shaker screens (175 mesh) were run on the shale shakers and the centrifuges were processing about half of the mud flow rate. Particle size analyses conducted on site while drilling indicated that the fines content (sub 4 micron) in the mud doubled from 14.9% on day three at a depth of 1148ft to 27.5% on day thirteen at a depth of 5200 ft. The formation encountered in this area are known to be highly bentonitic and therefore sensitive to degradation in fresh water mud systems. The increase in the amount of fines was thought to be due in part to the mechanical degradation of the bentonitic particles subjected to shear through the centrifugal pumps and mud pumps. To address the removal of fine solids and the elimination of sources of shear on the fragile clay particles, a solids control system without hydrocylones gas designed. Extra centrifuges were added, capable of processing the entire mud flow rate. This scheme lay two advantages; (1) no liquid would be dumped to the reserve pit and, (2) that it should be possible to maintain a lower mud weight and realize some increase in the penetration rate. Two questions arise. How much increase in ROP can be expected? And what is the realistic minimum mud weight that can be maintained without dumping fluid?
A simulation was conducted of the surface hole as it was drilled using the precise conditions that were used in the field including mud properties, bit selection, weight-on-bit, rotary speed, solids control program, and flow rate. Predicted rates of penetration from this simulation were then compared to actual penetration rate data from the rig.
Another simulation was done varying only the solids control system and keeping other parameters identical to the first simulation. Aa diagram of the solids control system configured for the second simulation including four shale shakers and five high volume centrifuges. A comparison is presented of the simulation results for bits 7, 8, and 9. The resultant effect of improving the solids control system on the ESD was that mud weight could be maintained at a minimum weight of 8.8ppg and that this reduction increased the penetration rate over 7% when compared with the 9.2 to 9.3ppg mud system. The additional cost of renting the five centrifuges for the 18 day duration of the surface hole would be 19,000 dollars. Daily operating costs in the Anschutz Field were about 18,000 dollars daily. The ESD indicates that the increase in ROP alone would justify the cost of renting the additional centrifuges. The most significant savings however is the reduction in reserve pit cleanup costs. With the five centrifuge system no fluid would be dumped. ESD results from the initial simulation indicate that underflows from the desander and the desilter alone would put about 700D barrels of fluid into the reserve pit. This plan realizes savings of approximately $28,000 solely from reducing the amount of effluent in the reserve pit.
This plan, when implemented, did achieve the major goal of reducing the effluent to reserve pit to less than 750bbls. (This compares to an average of 25,000 bbls on other wells in the area.)
This example demonstrates how the ESD accurately quantifies the interaction between between major systems in the drilling process. Consideration of the increase in penetration rate and the decrease in location clean-up expense indicate that the $19,000 rental expense of the extra solids control equipment is economically warranted for this example. In other geologies and drilling environments (where disposal costs are low) it might be shown that this system would be uneconomical. To make the optimum decision in either case requires the study of all attributes of the drilling system that is possible with the ESD.