A major challenge of Arabian heavy‑oil development is the problem of generating competitive advantages through deploying technological innovations while making cost-effective solutions a crucial part of a firm’s strategy for rigless interventions. The purpose of this paper is to examine coiled-tubing (CT) -stimulation and -logging technologies used in the timely project execution of one of Saudi Arabia’s largest field developments to enhance matrix-stimulation success in a cost-effective manner.
Introduction
The Manifa field is located in Saudi Arabia and measures approximately 45 km in length and 18 km in width, encompassing both onshore and offshore areas with water depths between 4 and 6 m. It was discovered in 1957, with first sustained production in 1964. However, because of low demand, the field was shut down in 1984. In 2006, a field-redevelopment plan was initiated with major capital spending. Drilling and development started in 2010.
New technologies have been enablers for the unique development of the Manifa field. To cover operations in the field adequately, 27 man-made islands were developed and connected by causeway. Manifa contains the largest number of extended-reach wells (ERWs) in the world; two-thirds of the wells in the field are ERWs by industry standards, some with a total depth (TD) of 37,000 ft, departing beyond 26,000 ft from surface locations.
The Manifa reservoir has five main layers. Two of these reservoirs targeted for development contain both heavy oil and relatively lighter crude. Generally, Manifa crude is heavy and sour (29 °API, 14% hydrogen sulfide in gas and 3% in oil). Production opportunities exist in the presence of formation heterogeneity and a relatively less-mobile-to-immobile heavy-oil layer between the aquifer and the zones above it. The reservoir has relatively low permeability. The viscosity of the heavy-crude layer affects the ability of the natural waterdrive to energize the flow of oil from below. Therefore, peripheral power water injection above the viscous layer was planned for the field to provide pressure maintenance and efficient sweep of hydrocarbons to the producers. Both injectors and producers are drilled with an overbalanced water-based-mud (WBM) system and are completed barefoot. In this field, the porosity is generally high, but the driving factors for a successful injector or producer placement include permeability and formation-fluid mobility, both of which vary significantly across the reservoirs. With heavy oil in place, the challenge was to place the horizontal-injector-well paths in mobile-fluid layers as close as possible above the nonproducible-heavy-oil interface, to ensure meeting injection-rate targets and to provide an optimal sweep of the lighter crude and minimize bypassed oil.
Apart from well placement of the long megareach horizontal single- and dual-lateral wells, another challenge was to conduct effective stimulation. In Manifa’s barefoot injector and producer completions, even relatively shallow drilling-induced near-wellbore damage can impede flow or injectivity substantially. Therefore, acid stimulation was required to remove drilling-induced reservoir damage caused by the overbalanced WBM, which uses calcium carbonate as weighting material.
Matrix-stimulation treatments were executed in the Manifa field with CT for accurate placement to remove drilling-induced damage. The extended-reach and openhole nature of the wells not only exacerbated the CT-reach problem but also posed challenges for stimulation-treatment placement. The challenges of CT stimulation entailed the need for multilateral access, real-time temperature monitoring to ensure optimized placement of stimulation fluids, and profiling to evaluate zonal contribution after stimulation. CT extended-reach solutions involved an integrated use of tapered CT strings (designed specifically for Manifa ERWs), drag reducers, and a variety of tractors and vibrators. Combination of these techniques resulted in achieving four consecutive world records for CT reach in an openhole completion (28,257, 29,897, and 30,365 ft, and, more recently, 37,000 ft).
Optimizing CT Reach, Multilateral Access, Stimulation Treatment, and Profiling
Solutions applied thus far to meet intervention challenges include the application of fiber-optic-enabled CT for acquisition of real-time bottomhole parameters during CT intervention. This CT system is equipped with four fiber-optic cables installed in a fiber carrier that allows pumping and ball drops with nominal CT pumping rate. CT-reach design assisted by simulations has facilitated efforts to maximize the extended-reach capability of the CT deployed in ERWs [i.e., before CT intervention, a simulation (with a tubing-force simulator) is run to predict how far the CT can reach and how much weight can be applied safely on the downhole end of CT]. The results of simulations depend on CT configurations such as size, grade, and wall thickness.
Several techniques have been applied to increase the lateral extension of CT into the open hole for effective stimulation treatment or to evaluate the effectiveness of the treatments. These techniques include the use of friction reducers or organic solvents to reduce encountered friction and the application of external forces by use of tractors and vibrators. A friction reducer creates a low-friction film on CT and tubing and formation surfaces and reduces drag forces (normal contact force between CT and completion). Tractors provide positive pulling force while running in hole, enabling the CT to reach the well’s TD. For a discussion of friction reducers, as well as other methods used to optimize CT reach, please see the complete paper.
The Manifa field provides evidence that the ability to control CT-access tools from the surface has also helped with multilateral-well re-entry, through window identification of the chosen lateral before re-entry. Through a downhole pressure-log display, confirmation of a successful entry into the lateral is possible. Horizontal-well intervention poses increased difficulty and a significant challenge, especially from the perspectives of zonal coverage and treatment effectiveness. Stimulation-treatment optimization has been demonstrated, especially in Manifa’s extended powered water-injection wells, yielding a consistent injectivity increase with a true reflection of stimulation-fluid penetration deep into the damage zone.
Need for Real-Time Bottomhole Parameters During CT Intervention
Technological improvements in horizontal drilling have resulted in complex wells for maximum reservoir contact. As the use of CT increases to access these wells for intervention purposes, the need for knowledge of downhole parameters has also increased. Fiber-optics-enabled CT likely provides the best CT-intervention option available in the oil and gas industry to date. This is a real-time system applicable for measuring real-time downhole parameters. The system consists of fiber optics inside CT connected to a pump through a bottomhole assembly (BHA). The fiber optics acts as a source of telemetry from downhole tools to the surface in real time and acts as a source of temperature measurement known as distributed-temperature sensing (DTS).
CT-Reach Design Assisted by Simulations
From examining approximately 100 planned ERW deviation surveys, wells were categorized as Category 1, 2, 3, 4, or 5 on the basis of the ends of the open hole or TD. Category 1 wells refer to wells with TDs of less than 14,400 ft; Category 2 refers to wells with TDs between 14,400 and 17,600 ft; Category 3 refers to wells with TDs between 17,600 and 20,800 ft; Category 4 refers to wells with TDs between 20,800 and 24,000 ft; and Category 5 refers to wells with TDs greater than 24,000 ft. CT-run simulations were conducted to predict CT reach to well TDs. Optimization of CT-string-designed sections (wall thickness and lengths) was performed to enhance CT reach. Wall thickness of CT sections is significant in simulations and job execution because the heaviest section, or the maximum wall thickness, of CT ideally needs to be in the vertical section of the well. In an ideal configuration, the medium wall thickness needs to be at approximately 60° well deviation and the smallest wall thickness needs to be in the well’s horizontal section. The idea of such a CT configuration is to ensure the transmission of forces from heavier sections to lighter sections of CT that maximizes the chances of CT reaching maximum depths. The objective is to have larger wall thickness in locations of maximum compression.
Thus, once optimization of the aforementioned CT parameters is achieved for one well, the model is validated with simulation software and applied to the remaining wells of a similar kind.
CT Access to Multilaterals
To meet the challenge of entering into each lateral, a multilateral tool (MLT) was used. The MLT system consists of an orienting tool (OT) and a controllable bent sub (CBS). The MLT can be operated by pumping fluid from the surface. The fluid sends a pressure signal to the surface and to a pressure gauge at the bottom of the hole, confirming that the correct lateral has been accessed. The ability to adjust the OT and CBS from the surface and obtain real-time bottomhole parameters saves considerable time during the mapping process.
The MLT is operated with the assistance of software that enables the display of several essential parameters, such as tool orientation relative to the lateral window. The software not only shows the current index or MLT profile mapped for the window but also indicates previous indices and guides the operator through indexing cycles, thus providing accurate real-time information on the downhole situation. After profiling the window, the software memorizes the window orientation and monitors the BHA orientation throughout the entire operation.
Stimulation-Treatment Optimization
CT tractors were introduced to pull the coil beyond the original lock-up depth, primarily to conduct matrix stimulation in Manifa’s ERWs. DTS technology was applied to improve the stimulation-treatment effectiveness. Record-breaking results reached with tractor-aided stimulation include the openhole-horizontal-stimulation record of 30,365 ft, with proof of significant improvement in injectivity and reduction in formation damage. The use of downhole pressure and temperature sensors in the CT BHA, and application of the fiber itself as a temperature sensor across the length of the well for DTS, enabled engineers to identify thief zones with a high degree of confidence. DTS measurements and interpretation helped to ensure diversion of the thief zones by spotting the appropriate volume of the diverting acid across the identified thief zones. Thus, engineers could make real-time decisions on-site on the basis of downhole measurements (as opposed to assumptions and rules of thumb).
Deploying real-time downhole readouts of temperature and pressure gauges by CT has distinct advantages because of the possibility of conducting and analyzing pressure-transient tests while operating. In produced-water-injection (PWI) wells, a common practice is to evaluate well injectivity before and after stimulation treatments. Therefore, a technique has been integrated to adjust the injectivity part of an injection/falloff test. Conceptually, a successfully treated PWI well should yield either a higher injection rate at less pressure (or equal) or the same injection rate at much less pressure. This means the stimulation objective is to reduce the formation resistance to the injected water from the surface.
Real-Time Production and Injection Profiling
The technical challenges posed by optimum placement of stimulation fluids and treatment evaluation were addressed by deploying a fiber-optic motor-head assembly (MHA). This MHA was an innovative solution to enable passing the fiber-optic cables through a standard MHA, then through the tractor to reach the optical-logging head, and finally to the production-logging tool (Fig. 1 above), enabling real-time measurement on the surface in the ERW.
This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 25119, “Saudi Arabia’s Manifa Giant Offshore Field Development: The Role of Technology,” by James Arukhe, Shadi Hanbzazah, Abdulrahman Ahmari, Saleh Al Ghamdi, and Karam Yateem, Saudi Aramco, and Moustapha Bal, Danish Ahmed, and Fernando Baez, Schlumberger, prepared for the 2014 Offshore Technology Conference, Houston, 5–8 May. The paper has not been peer reviewed. Copyright 2014 Offshore Technology Conference. Reproduced by permission.