Slim Hole Drilling Technology Research And Application

Compared with conventional wellbore, slim hole drilling may reduce the drilling cost significantly and has therefore been used more and more extensively in deep wells with complex structure and for drilling operations such as window drilling and sidetracking in old wells. However, there are challenges associated with drilling a slim hole, such as small annular clearance, great pressure loss, tendency to collapse and circulation loss. The slim hole drilling fluid technology has also posed challenges to the drilling process, for instance, slim drilling requires the drilling fluid to have good shear thinning behavior, flowablity and lubricating performance, as well as an outstanding ability to inhibit wall-building, prevent collapse, suspend the carry the cuttings generated during drilling. In this paper, investigation and studies have been carried out on the current slim hole drilling fluid technologies both at home and abroad and the technical challenges with slim hole drilling have been analyzed in a systematic way to determine the drilling fluid issues to be resolved: cuttings suspension and carriage, issues occurring under high density conditions, issues with lubrication, ticking prevention and reservoir protection. The film-forming polyer drilling fluid system with good inhibiting property and reservoir protection performance has been selected from the commonly used systems to serve as the basic mud for study, with the objective to further optimize its performance and satisfy the needs of slim hole drilling.Through a series of tests, high-temperature viscosity increaser has been added to increase the system’s ability to suspend and carry the cuttings, plugging agent has been optimally selected to improve the system’s plugging performance, lubricant and surfactant with good compatibility has been selected in an optimal way for combined use to reduce the tension at oil-water contact and improve the system’s lubricating performance. In addition, ideal filling and temporary plugging technologies have been combined to further promote the system’s reservoir protection ability. Finally, the formula of high temperature resistant, film-forming emulsified drilling fluid system has been determined: 5.00%~6.00% bentonite+4.50%~5.00% sodium carbonate NaOH+0.10%~0.15%KPA+0.2%~0.4% high-temperature viscosity increaser GZN+2.00%~3.00% sulfonated phenolic resin+2%~3% high-efficiency plugging fluid reducer DYFT-1 +2.00%~3.00% high temperature and salt resistant, anti-collapse filtrate reducer KFT +2.00%~3.00% high-temperature plugging agent GFD-1+2%~3% isolating film CMJ-2+2%~3% ideal filling agent+1%~2% white oil+1% emulsifier(LHR-1)+1%~3% pipe-freeing resistance reducing agent +3.00% extreme-pressure lubricant ORH-030 +3.00% liquid lubricant YDRH-1. Laboratory evaluation shows that under high temperature conditions, the high temperature resistance, water based, film-forming emulsified drilling fluid is more stable and has better capacity in preventing collapse, carrying cuttings and providing lubrication. This system also features low filtrate loss and the dynamic permeability of the core can be recovered to a value greater than 90%, which enables it to reduce the contact tension effectively. In addition, it requires low pressure differential for flow back and reduces the water locking damage, thus achieving good protection of the reservoir. This drilling fluid has a better resistance against contamination and it is effective to treat HCO3-contamination with FeSO4.Finally, pilot experiment on the application of this system in the slim hole horizontal section has been performed in Changling Gas Field. Throughout its application in the field, complications such as obstruction or sticking of the pipe string during the trips have not occurred, the one-run success rate of intermediate completion and completion logging has reached 100%, the average hole diameter enlargement rate in the oil-producing interval has been≤10%, casing running and cementing have been completed smoothly, the skin factor revealed by field testing is relatively low and the gas testing production rate is high. Compared with well Changshen D Ping 2, an cumulative economic benefit of 5.68 million CNY has been contributed by the four completed wells in Changling fault depression, namely, wells Changshen D Ping 3, 4, 6 and 11. Excellent economic and social benefits have been achieved on these wells.