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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractParticulate material to be removed from a wellbore can have a vast range in size and density. Typical materials are formation sand, drilling cuttings and various fracture proppants such as resin-coated sands, ceramics, Bauxite and ultra light-weight materials. The characteristic size, shape and density of the particles greatly influence their dynamic behavior in flowing media. Terminal velocity, drag and gravity forces and shear stresses are affected by particle properties and the rheology of the circulation fluid.This paper presents the results of a solids transport study on four different density proppants with same particle size (20/40 mesh) and three different diameter particles ranging from 0.15 to 7 mm. The specific gravity of the proppants varied from 1.25 to 3.6. The tests were performed using a sophisticated flow loop. Findings indicate that particle density and size have a significant effect on the solids transport. For a given flow rate, higher density solids result in higher in-situ solids concentrations and lower wiper trip speed (the wiper trip speed is the coiled tubing pull-out-of-hole (POOH) speed) and reduced transport efficiency. The solids transport for different particle sizes is strongly influenced by wellbore deviation angle. In a near-vertical wellbore larger particles have the lower transport efficiency while in a horizontal wellbore the medium sized particles have the lowest transport efficiency.New correlations have been developed from the experimental data to predict solids in-situ concentration, solids carrying capacity and optimum wiper trip speed for these tested solids under a given operating condition.

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.

The properties of the circulation fluid have a fundamental effect on solids transport. The shear stress at the solids bed and liquid interface, for a near horizontal wellbore, plays the key role in transport of the solids. The flow regime, geometric combination of hole/coiled tubing (CT) and eccentricity, also effect the rheological state of the liquid and have a significant impact on the solids transport efficiency. There is a need to differentiate between the superior solids suspension capabilities of the liquid and its hole cleaning efficiency produced when it is in motion. The most important concept is that, the greater the solids carrying capacity a fluid has, the more efficiently the hole can be cleaned.The challenge that presents itself is that once the solids fall and form a bed within the wellbore, how can the solids be re-entrained and transported out of the hole? In this paper, solids transport studies with several bio-polymers were conducted with a sophisticated flow loop. These studies highlight that these types of fluids bring some advantages and disadvantages. The carrying capacity and suspension properties of these fluids are superior but were hindered by other geometric influences on the velocity profile. Solids entrainment and re-entrainment into the fluid, as would be expected, is difficult to achieve without mechanical assistance.However, excellent efficiency of the fluid can be obtained and this paper presents some of the conditions under which this is practically achieved. Introduction For a typical fill cleaning process, the CT tags the top of the fill, and is run into the hole to a target depth while jetting into the solids (penetration stage). The hole is cleaned by; either circulating a clean fluid while keeping the CT stationary (circulation stage); or by pulling the CT out of the wellbore with continuous circulation (wiper trip stage); or by a combination of these stages. In past studies[1–4], several fluids have been used to conduct solids transport tests. Fluids previously examined are: water; 0.25% (by weight) Xanvis and 0.25% HEC gels. Based on these studies the following conclusions have been drawn. Horizontal Flow: The properties of the circulation fluid have an effect on solids transport. The shear stress at the bed interface for a near horizontal wellbore plays the key role in solids transport. Therefore, the flow regime, geometric combination of hole/CT and eccentricity also affect the rheological state of the liquid and have a significant impact on solids transport. Furthermore, there is a need to differentiate between the carrying capacity of the liquid and the hole cleaning effect produced by the flow. A consistent conclusion from the published references indicates that for a horizontal/near horizontal wellbore, hole cleaning is more efficient if a low viscosity fluid is pumped in a turbulent flow regime rather than a high viscosity fluid in a laminar regime. These previous studies are consistent with this trend and included a comparison of water, 0.25 % (by weight) HEC and 0.25% Xanvis polymers. The amount of solids that can be transported by a given volume of liquid is dependent on the rheological properties of the liquid. Xanvis and HEC polymer based fluids are more effective than water in terms of carrying capacity but unable to efficiently erode a stationary bed. (It is essential to keep in mind that CT is usually circulating the cleaning fluid at ‘low’ flow rates). In general, it is not possible to achieve in-situ velocities in a casing or open hole that are high enough to exceed the critical deposition velocity.

Thorough analysis of the many circulation parameters and force conditions experienced with coiled tubing has helped to improve job efficiency and eliminated many of the operating difficulties experienced in the past.The arrival of horizontal well technology has produced the need for more diversity and greater reliability from coiled tubing. Many standard completion and workover operations now require the use of coiled tubing in order to gain access to the horizontal sections. Using recognized correlations for all of the circulation parameters involved, and incorporating them into a set of force balance equations, a completely interactive model has been constructed to analyze flow conditions and tubing force effects and predict the outcome of coiled tubing operations. This paper will describe some of the concepts used to develop a coiled tubing simulator, its operating features and a few horizontal well examples to demonstrate its capability.Offshore Technology Conference, Houston (May 1989) OTC Paper 6037.