Fills Cleanout with Coiled Tubing in the Reverse Circulation Mode

Abstract

Abstract Removing sand fills from wellbores is one of the major applications for coiled tubing (CT). For the fills cleanout process, the fluid could be circulated in two different modes: forward circulation and reverse circulation. In the forward circulation mode, the carrying fluids are pumped through the CT down to bottom and flowed back to surface in the CT/casing annulus. For reverse circulation, fluids are pumped down the CT/casing annulus and back up the coil. In many cases with large completions and low reservoir pressures, the forward circulation mode cannot effectively clean the fills out of the annulus with only a limited available flow rate or without circulating expensive gel fluids, especially for the heavier particulates in deep and/or highly deviated wells. Due to the smaller flow areas and therefore higher velocities, fills may be more readily transported inside the CT flow channel. Therefore, reverse circulation may be an option to overcome the above constraints associated with forward circulation. Several major risks for reverse circulation with CT include coil collapse, loss of well control and sand bridging in the coil. Controlling the rate of penetration (ROP) of the CT into the fill is very critical for the reverse circulation sand cleanout process. There is a lack of knowledge on the effect of as well as optimum ROP during reverse circulation. In this paper, the maximum ROP for different sand types at different deviated angles and water flowrates for reverse circulation cleanouts are investigated with a full scale flow loop test facility. Based on these test results, empirical correlations have been developed and incorporated into an existing proprietary solids transport computer algorithm, which can now be used to optimize the hole cleaning process for both the forward and reverse circulation modes of solids transport. Introduction Several wellbore cleanout methods have been developed over the years. One of the most common operations is running in with coiled tubing and circulating the solids out with a liquid or multi-phase fluid through the annulus between the CT and wellbore (i.e. forward circulation, Fig. 1). Solids tend to settle and form an equilibrium bed on the low side of the wellbore in highly deviated or horizontal wells. This problem is exacerbated by the eccentric annular flow path created with the CT entering deviated wellbores. Fluids with enhanced solids suspension properties tend to have poorer solids reentrainment abilities once a stationary solids bed has formed. The conventional approaches to removing the solids bed involve using higher flow rates, or employing exotic and costly fluids, neither of which ensures complete fill removal in every case. Based on prior comprehensive research1–6, an effective CT sand cleanout methodology utilizing the forward circulation mode has been developed, patented7, and verified during numerous field operations8–13. This proven sand cleanout process entails a down hole wash tool and a sophisticated computer based methodology for CT deployment in vertical, deviated and horizontal wells. The preferred down hole tool incorporates fluid nozzles with switchable forward and backward facing jets. By selecting the backward facing jets and controlling the pull-out-of-hole (POOH) speed (at rates determined by the associated particle transport software), the settled sand bed can be "swept" out of the hole with near 100% efficiency. However, in some cases involving large completions in deep and/or highly deviated wells, the forward circulation mode cannot effectively clean debris fills out of the annulus with the limited flow rate available or without resorting to expensive gel fluids8. Due to the smaller flow areas and higher velocities, fills can be effectively transported by the internal CT flow channel. Therefore, reverse circulation presents an alternative clean out mode that could overcome the above limitations associated with forward circulation.