Cleaning debris from a wellbore is a common operation with coiled tubing (CT). Considering that this process is a complex function of fluid properties, flow velocities, hole size, deviation angle and particle properties, it becomes particularly challenging in wells with low bottom hole pressures (BHP). There are two circulation modes involved in conventional sand cleanouts with CT, namely forward and reverse circulation. Either sand cleanout method may apply excess hydrostatic pressure on the formation that can result in loss of circulation and returns, hence making cost effective solids removal impossible as well as potentially damaging the formation. In conventional cleanouts, nitrogen or low density hydrocarbon fluids can be used to reduce hydrostatic pressure; however this necessitates a very careful job design and execution which can require expensive volumes of nitrogen (N2) or hydrocarbon fluids. An alternative technology that combines a concentric coiled tubing (CCT) string with a down hole jet pump to remove solids from a wellbore but without placing any additional hydrostatic pressure on the formation has also proven to be remarkably successful. This paper reviews these different solids cleanout methodologies and summarizes the advantages and limitations related to each method when used in low formation pressure wellbores. New solids transport flow loop test results, related to a hydrocarbon fluid, are summarized and compared with tests using water. Case histories are presented that demonstrate how to select the most appropriate cleaning method based on well conditions. Introduction Sand cleanouts with coiled tubing have been performed for many years and 30 to 40% routine services performed in the coiled tubing industry entail sand cleanouts. In most cases, simply due to the presence of sand in the wellbore, the production rate decreases requiring the sand to be removed. In other cases, it is also a known fact that many CT operations require a cleanout before the main well workover operation can commence-3. Several cleanout options have been developed over the decades employing a number of different approaches and techniques 1. Wellbore cleanouts with coiled tubing or conventional jointed pipe often incorporate high circulation rates, special fluids or reverse circulation mode to remove solids. With high rates and high specific gravity water-based fluids, conventional sand cleanout methods excert excess down hole pressure on the formation that can result in lost circulation of returns in the low formation-pressure reservoirs. This makes sand removal impossible and creates damage to the formation. Nitrogen can be used to reduce hydrostatic pressures, but this necessitates a very specific job design and execution and can require massive amounts of liquid nitrogen in the case of horizontal wells that can create further logistical problems when these are located in some remote area. The evolution of coiled tubing technology has brought a unique opportunity for a solution for this problem. Sand/well vacuuming technology has been developed, patented, and proven by field operations to readily clean out a wellbore with low pressure. The sand/well vacuuming system consists of a specialized down hole jet pump connected to a CCT string. The tool can be operated in two modes: sand vacuuming and well vacuuming. The tool induces a localized drawdown in pressure as it passes any point in the wellbore, which effectively removes flow-obstructing sand in the sand vacuuming mode or localized mud damage debris in the well vacuuming mode. The selection of sand cleanout methods must be based on both logistical and technical issues. Equipment costs, reel weight, availability and/or costs of N2 etc. may be the deciding factors for some sand cleanout operations. Technical considerations typically include formation damage potential (i.e. production engineers may not want to pump gel in certain regions), low BHP and/or small completion tubular (i.e. insufficient circulation with N2 required for conventional cleanout systems) and particle size and/or type of debris.

Annular fracturing of multiple zones with coiled tubing (CT) has been practiced for more than a decade. A typical treatment involves sand jetting perforating, cleaning the well, fracturing and isolation. It is economically desirable to perforate, fracture and isolate each interval quickly, allowing all intervals to be treated in the shortest time possible. However, CT lying on the bottom of horizontal wells creates a solids bed during the perforation and fracturing stage, this introduces additional complexities into the reliable execution of the fracturing process. For tight formations, proppant beds have to be completely or partially removed, otherwise the pad can entrain the proppant which creates a high potential for premature screen out. In this paper, a new approach to cleaning the hole between multiple annular fractures is proposed. This hole cleaning process involves circulating the clean fluid through the annulus to the bottom of the wellbore and bringing the sands back to the surface through the CT; at the same time the CT pulls out of the hole (POOH). The sand particles at the top section of the wellbore are swept toward to the end of CT, while the solids near the end of the bottom hole assembly (BHA) are sucked into the CT as it is pulled out of hole. Eventually, all solids are removed with the CT partially pulled through the initial bed section. As a wiper trip to surface is not required, the efficiency of this new process is higher than other cleaning processes. A full scale flow loop was used to simulate the process. Maximum POOH speed and the hole cleaning time were determined under various conditions. Empirical correlations were developed to allow for optimization of the process under the down hole conditions.

Annular fracturing of multiple zones with coiled tubing (CT) has been practiced for more than a decade. A typical treatment involves sand jetting perforating, cleaning the well, fracturing and isolation. It is economically desirable to perforate, fracture and isolate each interval quickly, allowing all intervals to be treated in the shortest time possible. However, CT lying on the bottom of horizontal wells creates a solids bed during the perforation and fracturing stage, this introduces additional complexities into the reliable execution of the fracturing process. For tight formations, proppant beds have to be completely or partially removed, otherwise the pad can entrain the proppant which creates a high potential for premature screen out.In this paper, a new approach to cleaning the hole between multiple annular fractures is proposed. This hole cleaning process involves circulating the clean fluid through the annulus to the bottom of the wellbore and bringing the sands back to the surface through the CT; at the same time the CT pulls out of the hole (POOH). The sand particles at the top section of the wellbore are swept toward to the end of CT, while the solids near the end of the bottom hole assembly (BHA) are sucked into the CT as it is pulled out of hole. Eventually, all solids are removed with the CT partially pulled through the initial bed section. As a wiper trip to surface is not required, the efficiency of this new process is higher than other cleaning processes. A full scale flow loop was used to simulate the process. Maximum POOH speed and the hole cleaning time were determined under various conditions. Empirical correlations were developed to allow for optimization of the process under the down hole conditions.