Development of a Downhole Separator and Intensifier for Coiled Tubing Jetting

Abstract

Abstract Wells often build scale deposits on the inside of their tubulars that can impede production or interfere with a workover operation. When this scale buildup is over long intervals, coiled tubing (CT) is often the conveyance system used to deliver acid, drilling or jetting technologies to remove the scale. Acidizing through coil can be effective only if the scale is acid soluble and it can be expensive. Drilling with motors can be very effective, but motors are susceptible to performance issues, especially if high nitrogen ratios are required. Jetting the scale is possible, but this technology also faces challenges. Delivering sufficient downhole hydraulic pressure to overcome the scale's threshold pressure is very difficult due to the limited pressure capacity of CT and frictional pressure losses in long CT strings. Further, if nitrogen is required to ensure returns, the gas phase quickly disperses the fluid jet as it exits the jetting tool, and very little power is delivered to the scale face. Recent technical advances in CT jetting technology include a rotary separator to separate the nitrogen from the fluid and a downhole pressure intensifier to take the separated water and increase the hydraulic pressure delivered to the jetting tool. The paper first discusses measurements of the threshold pressure required for removal of oilfield scales. The development of a gas separator and downhole intensifier are discussed next, followed by the results of testing these tools. Introduction Mineral scale deposits inside of production tubing can reduce production or interfere with workover operations. Coiled tubing is used to remove this scale using motors or jetting tools. Jet milling of scale is an attractive option because fluid jets will not damage tubulars or other downhole equipment. Jet milling capabilities are limited by the threshold pressure required to initiate milling and by fluid jet dissipation. The jet pressure delivered to the scale surface determines the ability of the jet to cut a given scale. The jet power then determines the rate at which the scale can be removed. The pressure that can be delivered to a jetting tool through CT is limited by fatigue limits of the coil and the pressure capabilities of available pumps. The jet power, which determines milling rates, is further limited by friction pressure loss in the CT. These losses become more significant in deep wells. One approach to cutting scale at the pressure available using coil is to add abrasives(1), however, this approach adds cost and complexity to the operation. Another approach is to boost the pressure of the jets with a downhole intensifier. In the 1990s, the FlowDril Corporation manufactured a large-scale downhole intensifier pump to drill 77/8 in to 83/4 in holes with jet assisted roller cone bits(2). The unit was designed to work with a conventional rotary drill string and run on drilling mud. The intensifier ratio was 14:1, delivering 84 l/min at 200 MPa from mud supplied at 1,260 l/min and 23 MPa. A similar approach would allow CT pressure to be boosted to a level capable of milling hard scale. The effectiveness of fluid jetting tools is further limited by dissipation of the jet in the high pressure well environment. Submerged, non-cavitating fluid jets are subject to rapid dissipation due to turbulent mixing of the fluid. The maximum length of the high pressure jet core produced by an ideal jet under these conditions is under seven nozzle diameters(3). Intense turbulence persists to a range of around 20 nozzle diameters. By contrast, water jets in air can be effective at ranges of over 1,000 nozzle diameters. Jet dissipation effects