Integrating geomechanics improves drilling performance

Shale plays have been a proving ground for breakthrough technologies as the industry continues to recognize just how complex these unconventional reservoirs are. As operators push for multistage fracturing in deeper, longer laterals and expand into new areas globally, they are learning that integrating efficient drilling practices with geomechanics is critical for addressing the unique challenges that shale gas and oil wells can present.

FIGURE 1. The curve section was drilled with a DLS using the PowerDrive Archer RSS that builds high angles from any deviation. (Images courtesy of Schlumberger)

Among the obstacles encountered when drilling horizontal shale wells are wellbore instability and lost circulation. Shales generally exhibit high degrees of heterogeneity in terms of the mechanical behavior. Most organic-rich shale formations are overlain by clay-rich mudstone with thinly bedded laminated structures, resulting in anisotropic rock strength and horizontal stresses.

Wells drilled oblique to these bedding planes over a certain deviation have severe instabilities in the build section due to exposure of planes of weakness. The presence of natural fractures makes the well even more difficult to drill as the hole can cave in easily. This can potentially result in hole pack-off and stuckpipe incidents.

These conditions lead to problems that impact the efficiency of the operation and nonproductive time, consequently increasing the cost. The higher the well deviation, the higher the required mud weight (MW) to avoid stuck-pipe incidents stemming from wellbore instability. If MW is increased above the fracture gradient, the formation can fracture, or existing natural fractures can reopen, resulting in partial or total lost circulation.

This phenomenon is particularly problematic in the lateral section, where the safe mud window is typically narrower than in the build section. Even if the MW is managed properly within the allowable limits, any surge in equivalent circulating density (ECD) due to ineffective drilling practices can lead to losses, making it more difficult to achieve a good cement bond for the casing or liner and jeopardizing the fracturing job.

By effectively integrating geomechanics with efficient drilling practices throughout the well construction process, Schlumberger has optimized well design in shale wells, enabling operators worldwide to reduce wellbore instability and reach planned total depth (TD) without incurring losses.

Optimizing well construction

Lessons learned from the successful application of an integrated geomechanics approach in North American shale gas plays helped a major operator in the Middle East optimize well construction and make important adjustments to the drilling plan in its shale gas wells. By taking a multidisciplinary, integrated approach to analyze the reservoir with advanced technology, Schlumberger partnered with the client to significantly improve the efficiency of the entire operation and save considerable cost and time.

The drilling plan initially called for entering the formation above the target zone at 77° to land the horizontal in the required target zone. The predrill mechanical earth model (MEM) and wellbore stability model predicted several unstable intervals or problematic zones directly above the target formation, which could potentially lead to hole pack-off or stuckpipe incidents if drilled at that angle.

The predrill wellbore stability model ran several simulations that determined that a lower inclination in the problematic zone would be the best option for providing the optimum MW window and minimizing wellbore instability caused by thin layering of the shale. The trajectory was optimized and the plan revised to enter the problematic zone at a 65°and exit at a 73° deviation, which did not leave enough room to build the rest of the angle to land the well within 4.6 m (15 ft) below the problematic zone unless drilled with a high dogleg severity (DLS).

To address that issue, the curve section was drilled with 10° per 30.5 m (100 ft) of DLS, reducing the directional work required and minimizing footage drilled across the problematic zone. This higher dogleg curve was achieved with the PowerDrive Archer, a high build-rate rotary steerable system (RSS) that builds high angles from any deviation. The technology can drill through hard interbedded formations, extending the distance of horizontal sections and allowing casing to be run more easily and at the immediate exit of the problematic zone.

As part of the trajectory optimization process, 9 5/ 8 -in. casing was set in the vertical pilot hole at a pre-planned depth, and an 8 3/ 8 -in. hole was drilled and plugged back after logging. The high build-rate RSS was used to sidetrack, without using a whipstock, out of the cement plug to drill the horizontal section. The final optimized well trajectory with less severe deviation angle across the problematic zone and high-DLS design provided a safer MW window, which allowed the operator to drill the target zone at a lower MW and avoid any potential drilling-induced fractures or lost circulation. This solution allowed the operator to eliminate one casing string and drill the curve section in less than a week, representing a time savings of 15 days per curve section compared to initial plans.

Real-time geomechanics

FIGURE 2. The high-DLS design provided a wider mud window in the curve section of the well.

In the lateral section, the key objective was to manage ECD and drill to planned TD. The MEM and wellbore stability models predicted a narrow mud window with a lower lost circulation MW limit. The presence of natural fractures complicates the situation as these fractures can reopen even with pressures lower than fracture gradient and lead to lost circulation. Sweep performance was closely monitored, reducing the pill usage. Pumping after every alternate stand instead of every stand helped increase cuttings recovery, and mud treatment (reducing drill solids) resulted in an immediate reduction in ECD, which was the key in managing the lost circulation.

To monitor hole condition and manage ECD, 24/7 real-time geomechanics support aided the operator in acquiring LWD sonic data to update the geomechanics model in real time to better estimate stresses and fracture gradients and also predict the MW window ahead of the bit while drilling.

KLA-SHIELD, a high-performance water-based drilling fluid with inhibitive chemicals designed not to react with shale, was used in the lateral, which enabled acquisition of a high-resolution LWD resistivity image and better understanding of the natural fractures network. This allowed the operator to continuously monitor borehole conditions and make key changes to drilling procedures going forward.

The deeper understanding of this unconventional reservoir was gained through the use of geomechanics to enhance more effective shale well planning and construction and promote better application of drilling practices and technologies that improved operational efficiency. The time savings for the optimized well design compared to the conventional design was 15 days. Following this successful operation, the operator applied the same approach in six additional wells in the region, with no wellbore stability incidents that compromised the integrity of drilling and completions operations.

Acknowledgment

This article was adapted from SPE/IADC 166735, which was presented at the Society of Petroleum Engineers/International Association of Drilling Contractors Middle East Drilling Technology Conference and Exhibition in Dubai, United Arab Emirates, Oct. 7 to 9, 2013.