High build rate RSS sets records while targeting thin reservoirs

DENG XIANG BAI, YONG SHENG,  YI ZHU, QIANG LI, and QI WANG, PetroChina Tarim Oilfield Company; PHAN VAN CHINH, COLIN KE and MIKE ZHANG, Schlumberger

In recent years, technological breakthroughs in downhole tools, bits and integrated systems have dramatically enhanced operators’ ability to drill, land and steer complex wellbores in increasingly challenging environments. In early 2013, for example, PetroChina began drilling horizontal wells in a mature oil field in western China, using a hybrid rotary steerable system (RSS) capable of achieving higher build rates than conventional tools. Despite very thin, hard target reservoirs and high structural uncertainties, all three ultra-deep wells drilled with this new technology achieved their objectives while breaking existing performance records. As a result, PetroChina is reducing total runs per well, boosting rates of penetration (ROP), extending footage per run, minimizing nonproductive time (NPT) and costs, and increasing reservoir exposure.

HADE OIL FIELD CHALLENGES

Tarim Oilfield Company (TOC), a subsidiary of PetroChina Company Ltd, is engaged in prospecting, developing, and producing oil and gas from fields in the Tarim basin, one of the most prolific hydrocarbon producing regions in China. Located in the Xinjiang province of western China (Fig. 1), the basin is among the world’s most challenging areas to conduct exploration and development activities. Much of this remote basin is a vast desert, surrounded by high mountains. Hade (or Hadexun) oil field is one of four major producing fields in the northern Tarim basin, characterized by deep or ultra-deep reservoirs, with relatively high pressures and temperatures, moderate porosities and permeabilities, and low viscosities.

Fig. 1. The Tarim basin, one of the country’s most prolific producing areas, is in the Xinjiang province of western China. PetroChina aims to increase production to 30 MMtoe by year-end 2014.

Hade field is a low-relief structure, with a complex tectonic and depositional history resulting in subtle stratigraphic-anticlinal trapping and a complicated oil/water interface in places. Oil reservoirs include the main Donghe Sand member, and two thin sand layers located within the Middle Mudstone formation, at burial depths of more than 5,000 m, TVD. These thin target sands—the subject of this case study—consist of fine, tightly cemented clastics of volcanic and metamorphic origin, exhibiting rock strengths up to 15,000 psi.

In the center of the field, where reservoir thickness is greater, geological uncertainties are lower, and the geosteering challenge is not as complex, TOC relies on local directional drilling partners. However, the company generally turns to international drilling contractors to deliver horizontal wellbores at the boundaries of the field, where uncertainties are high and the target reservoir is only 1-3 m, TVD, in thickness, and sometimes even thinner. The hard, abrasive sandstone causes considerable shock and vibration, and a high rate of tool failures while drilling and steering the 8½-in horizontal section using conventional RSS technology. Due both to rig constraints—such as sub-optimal rotating speed—and RSS dogleg limitations, it proved difficult, and in some cases impossible, for drillers to achieve the required directional well profiles.

The high rate of equipment failures had two additional consequences. First, it led to significant non-productive time (NPT), partly because was it necessary to pull out of hole frequently to change tools. Three to five runs were often necessary to drill the build-up section, and outcomes remained unpredictable. Second, given the number of tool failures and the complex logistical situation, some international contractors lacked the local infrastructure or advanced drilling technology necessary, either to sustain or to improve performance in this extremely challenging environment.

When TOC began planning another horizontal drilling campaign at the edge of the structure in late 2012, Schlumberger, which has had full operational support infrastructure in the region for years, offered to bring a unique integrated solution to Hade field’s tough drilling and geosteering challenge. The objective was to maximize reservoir contact in the 8½-in lateral section, while mitigating risks and minimizing total drilling time and costs.

HYBRID RSS WITH CUSTOM BITS

For the 2013 drilling program, which began in January, the Schlumberger PowerDrive Archer high build rate RSS technology was chosen to target thin, hard sandstone reservoirs. This unique RSS, first commercialized in 2011, represents a breakthrough in steering control. It is the industry’s first truly hybrid RSS, integrating proven components from push-the-bit and point-the-bit systems.

In a conventional push-the-bit RSS, external pads push against the borehole wall, forcing the bit in the desired direction. On the other hand, a conventional point-the-bit RSS uses an internal bend to misalign the tool, relative to the borehole axis. Combining elements of these two systems, the new RSS provides higher dogleg capability and directional control than either tool alone, Fig. 2. All external components rotate fully with the drillstring. In steering mode, its electronic control unit holds a rotary valve geostationary, diverting a small percent of mud flow to a set of internal pads, which push on the inside of a stabilizer sleeve instead of the borehole wall. The sleeve is hinged on a universal joint, which pivots and points the bit in the desired direction. The degree of deflection can be controlled mechanically, and held electronically in a constant direction, as long as necessary, to achieve the objective.

Fig. 2. The PowerDrive Archer RSS combines proven push-the-bit capability (top) with point-the-bit technology (bottom) in a true hybrid steering unit, providing greater build rates and directional control than conventional RSS technology.

As with a hybrid steering system rotating through hard formations and higher doglegs, specialized bits and fatigue management are necessary to ensure reliability and optimum drilling performance. The high build rate RSS is part of an “engineered drilling system” consisting of a custom PDC drill bit from Smith Bits, drill string, and bottomhole assembly (BHA). Using the Smith Bits IDEAS integrated drillbit design platform (Fig. 3), engineers simulate and optimize custom bit behavior as part of the entire system, using a range of operating parameters specific to the well, formation and field.

Fig. 3. The IDEAS integrated drillbit design platform simulates bit behavior in the context of the entire drilling system—including the RSS, BHA and drillstring—optimizing performance and durability for specific applications and fields.

ECONOMIC VALUE / NEW RECORDS

Since first deployment of the hybrid RSS and integrated drillbit technology in January 2013, PetroChina Tarim Oilfield Company has finished drilling three horizontal wells targeting thin oil reservoirs at the edge of Hade field. The new tool has proven capable of achieving the required build rate despite the hard formation, while reducing shock and vibration and minimizing tool failures. After kicking off deeper than offset wells and landing in the reservoir about 50 m sooner, the 8½-in sections of these wells have been successfully geosteered in sands as thin as 0.3 m, TVD.

Application of this unique technology in Hade oil field has yielded a number of economically valuable benefits. Because the high build rate RSS is roughly half the length of a conventional RSS, and simpler to support and maintain onsite, the system has eliminated the logistical complexities and delays of shipping replacement tools or parts from elsewhere in China.

The high build rate RSS, combined with custom drill bits in an integrated BHA, can drill in a single run what previously required three to five runs with conventional technology—a 67%-to-80% improvement. These advanced drilling technologies have improved the average ROP in the 8½-in section from 2.65 m/hr in previous wells to 4.65 m/hr—a 75% acceleration. Additionally, the average footage in the 8½-in section has been extended from 236 m/run in previous wells to 503 m/run (Fig. 4)—a full 113% improvement.

Fig. 4. The hybrid RSS and custom integrated drill bits extended average footage in the 8½-in section from 236 m/run in previous wells to 503 m/run—a 113% improvement. The system also set a new field record: 620 m in one run (right).

Fig. 5. Time-depth curves for three wells drilled in 2013, using the new RSS, indicate a total reduction of 25 days compared with offset wells, saving PetroChina approximately eight days per well, due to drilling performance improvements.

CHALLENGING APPLICATIONS

Since its introduction in 2011, the high build rate RSS has proven effective at overcoming tough drilling challenges in a number of environments worldwide. In western China, the first application has been for thin, abrasive, ultra-deep oil reservoirs at the outer edges of a complex structure, with high uncertainties and associated risks. Elsewhere, drilling engineers have found applications in deepwater wells with unconsolidated sands, unconventional shale plays seeking maximum reservoir contact, and other fields in which high build rates and reliable directional control are critical to success. China, itself, has substantial unconventional shale resources, which may benefit from the technical capabilities of this advanced geosteering solution in the future.