Real-time Well-site Monitoring and Evaluation of Coiled Tubing Cleanouts
Authors: Walton Ian C. 
Published: December 2007

Abstract
This paper describes the development and application of a new method to monitor and evaluate coiled tubing sand-fill clean-out operations. The method provides real-time diagnostics at the well-site using downhole pressure data. Input data include well geometry and coiled tubing data and real-time data from a Downhole Sensor Package and surface sensors. The associated interpretive algorithm is not intended to simulate the clean-out process, but it is designed to obtain as much information as possible about the progress of a CT clean-out operation using only the sensor inputs. The use of mathematical models of the physical processes is kept to a minimum; in particular, an estimate of frictional pressure is made from the data instead of from a calculation. Application of the method is illustrated with data from simulations and field trials.
Use of this tool will improve the efficiency of the operation by detecting whether the fill is lifted efficiently (or deposited as a bed), by predicting when fill will arrive at the surface and by allowing the coiled tubing to be run out of hole as the last fill is lifted. An estimate of the volume of fill lifted may indicate the need for further circulation or an alternative method of treatment before the coiled tubing is pulled out of hole. Estimates of the depth of fluid originally in the well and its density are also provided, along with an estimate of the depth of the top of the fill.

Introduction
The ability to make downhole measurements while pumping fluids is a valuable asset in controlling the quality and maximizing the effectiveness of services provided by coiled tubing (CT). In the work described in this paper the downhole data is provided in real-time by a Downhole Sensor Package (DSP).
The Downhole Sensor Package (DSP) is a data acquisition system developed for use with coiled tubing which provides real-time measurement of down-hole pressure and temperature and detects the location of casing collars during stimulation and workover treatments. The tool system is run on the end of a CT string and data is transmitted to the surface via an electric line installed inside the CT. A conventional strain gauge is used to measure the pressure. A flow-by housing can be used to allow fluids to bypass the tool, thus allowing circulation below the tool. This is particularly useful for treatments such as sand fill cleanouts. The wire, installed inside the CT is either a mono-cable or hepta-cable as used to perform electric wire logging services. A software package developed for use with the system allows a display of downhole and surface parameters at the wellhead during the job.
The main advantages of the DSP are that it allows downhole measurements to be made in real time with a simple acquisition system while pumping fluids through the CT even in deviated or horizontal wells. Bottom-hole pressure (or, more accurately, the pressure at the end of the CT) is made available without recourse to calculations for hydrostatic head and frictional pressure losses. The value of using the DSP in CT operations generally has been discussed by Lemp et al while Acock et al have described its use in the real-time optimization of acid treatments.
The present paper describes the development and application of software to interpret DSP data during a sand-fill cleanout. The removal of sand or other fill from a wellbore is one of the major uses of coiled tubing. The operation involves the circulation of a fluid through the CT to the sandface where the sand is picked up by the jetting action of the nozzles. It is then transported to the surface through the annulus between the CT and production tubing or casing. A detailed numerical simulator of this process (WBS) has been developed over the last few years. P. 881^