Drilling wells with longer horizontal sections to maximize the reservoir contact has brought many challenges to the well interventions using Coiled Tubing (CT). In most cases, the complexity has pushed the CT interventions to the limits, driving the development of new solutions to extend the reach to the total well depth. The number of these extended wells has grown significantly in recent years in Saudi Arabia, especially in new development fields like the study area, where more than 100 extended reach power water injection wells were placed in the reservoir flanks. The water injection wells in this study area are a typical example of complexity due to the combination of large bore tubular, lower reservoir pressure, and long horizontal openhole completion, with lengths between 4,000 to almost 8,000 ft. During the initial CT interventions in this field, several challenges were encountered, e.g., stuck CT due to pressure differentials and slack-off weight through severe doglegs, tight spots, and other obstructions. Extensive knowledge was gained by performing an extensive simulation and study of the horizontal section geometry, and analyzing the operational data. Based on the findings, a combination of techniques was introduced to the CT intervention procedure, to extend the reach and minimize the operational problems and costs. The key components of our new implemented methodology were buoyancy reduction by using nitrified fluids and friction reduction by using new viscoelastic surfactant (VES) friction reducer. In addition, combinations of other techniques were employed, e.g., optimizing the pull tests frequency in openhole and improving CT movement practices. This paper shows the effectiveness of the new cost effective and reliable methodology to maximize the CT reach up to 8,000 ft allowing the distribution of the acid stimulation throughout the entire horizontal sections. INTRODUCTION This paper is based on the CT stimulation campaign performed in one of the new fields in Saudi Arabia that is being developed by drilling extended reach horizontal water Injection wells (Figure 1) in the flank of the reservoir to assure the effective water distribution, optimum sweeping and pressure support in the reservoir. Acid stimulation using CT is not an easy process due to the wellbore architecture, length of the horizontal section, reservoir hetrogentiy, low reservoir pressure, damage distribution, etc. For carbonate formation, the success of HCl acid stimulation depends on the placement and effective acid distribution along the entire openhole. Although CT represents the most effective placement method in a horizontal section, it has some limitation especially on extended reach wells with complex configuration. The definition of an extended reach well is a well with a measured depth to true vertical depth ratio (MD/TVD) equal to or greater than 2.

Relative permeability modifiers (RPM), hydrophobic in nature, are used to selectively reduce permeability to water with minimum impact on permeability to oil. Lab studies and field application reveal that RPM polymers, in some conditions, may damage the permeability to oil resulting in reduction/ or lost of oil production.Single and parallel core flow tests were conducted on carbonate and sandstone cores at reservoir conditions. Single core flow tests showed that RPM polymers can damage tight cores (less than 1 md) by forming an external filter cake on the injection face of the cores. Accumulation of RPM polymers at the face of the core means that these chemical will not properly propagate inside the core, which significantly reduces their ability to minimize water production.Parallel core-flood experiments conducted at reservoir temperature (200 o F) showed that RPM damaged the permeability to oil when there was a large permeability contrast (greater than 1:5) in favor of the oil-saturated core. Due to the high permeability contrast, most of RPM polymer solution preferentially entered the oil-saturated zone, thus damaged the oil production zone. Field data supported lab findings and showed that oil production of an oil well dropped significantly following RPM polymer treatment. It was found that polymer injection rate was a key factor to minimize damage to the oilsaturated core whenever we have a large permeability contrast in the favor of oil-saturated core. Injection of RPM polymer at low rate (1 cm 3 /min) was found to minimize damage to the oil-saturated core.Field results showed that application of water control treatment using RPM polymer on an oil well resulted in a significant loss in oil production. Post treatment analysis indicated that RPM polymer solution entered mainly the oil-saturated zone, which was attributed to the large permeability contrast in favor of the oil-saturated zone. This paper will present detailed lab studies and field data obtained and will recommend the proper RPM polymer application method to avoid damaging oil producing zones.