Saudi Aramco – Frack to the future

The history of fracking worldwide is long and storied, and over the past 60 years more than 1.1mn hydraulic fracturing treatments have been conducted that helped in delivering more than 600trn cubic feet (TCF) of natural gas. Saudi Aramco says that, today, 90% of all oil and gas wells are routinely fractured where moderate to tight reservoirs are encountered.

With the inception of its non-associated gas development program in 2000, Saudi Aramco introduced hydraulic fracturing technology to its reservoirs for enhanced hydrocarbon production rates and reduced sand production.

The process has continued and significantly increased in its use and hundreds of stages have been pumped with millions of gallons of acid and millions of pounds of proppants. Aramco says the technology has proven very effective, providing full well potential, maintaining long-term production sustainability, significantly cutting number developmental wells and commercialising tight reservoirs.

“There is a huge reserve of moderate to tight gas worldwide,” Adnan Al-Kanan, Gas Reservoir Management Department (GRMD) Manager, says.

“It requires fracturing to attain economic production and the tighter the interval is, the more challenging it becomes to exploit the hydrocarbon. The oil and gas industry is spending substantially to develop tight reservoirs through the use of hydraulic fracturing, and the number of fracturing stages is generally inversely proportional to reservoir permeability.”

Saudi Arabia’s non-associated gas reservoirs are predominantly encountered onshore in the Ghawar region as sandstone and carbonate intervals. The prolific offshore gas reservoirs with high permeability need little or no stimulation. Most of those wells are completed either open hole or with pre-perforated liners.

Hydraulic fracturing was a breakthrough for the success of Saudi Arabian gas programme, with the technology being applied in moderate to tight gas reservoirs. Dr Zillur Rahim, Senior Petroleum Engineer Consultant and Hydraulic Fracturing Team Leader, GRMD, at Saudi Aramco, says that subsequent to the first few stimulation treatments as trial tests, the fracturing technology started to become a routine practice to enhance gas rate in both carbonate and sandstone formations.

“With new fields being discovered with their associated challenges, Saudi Aramco also embarked on improving the fracturing technology and made much progress to optimise fracturing treatments with the use of high-strength proppants, retarded acids, low polymer clear fluids, advanced multi-stage completion assemblies, and innovative treatment designs,” he says.

“Starting with small scale fracturing treatments on the vertical wells during 2000-2007, today’s fracturing campaign are conducted on horizontal wells where special completion equipment is used and a single lateral is fractured in multiple stages.

“The fracturing intervals are assessed from the reservoir development captured by the openhole logs run either during drilling or shortly thereafter.

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Saudi Aramco has advanced considerably in horizontal drilling technology, sand control mechanisms, real-time application of geomechanics, customised completion assemblies, and application of novel fracturing fluids, proppants, and acids. These are some of the key technology items that were carefully optimised and implemented to make the gas programme successful.”

Use of such technologies ensured wellbore stability while drilling in the more challenging minimum horizontal stress direction, smooth deployment of the multi-stage fracturing assembly, and obtaining high conductive fractures due to the application of better fluids and proppant types. Hydraulic fracturing has resulted in improved productivity, optimum gas rate, and higher ultimate recovery.

“Numerical simulation has been integral in allowing us to model and compute the best drainage configuration, well placement, target intervals, and directional azimuth and fracturing characteristic scenarios,” says Dr. Hamoud Al-Anazi, Offshore Gas Reservoir Engineering Supervisor, GRMD, Saudi Aramco.

The fracturing intervals are assessed from the reservoir development captured by the open hole logs. The initial challenge of keeping each hydraulic fracture separate from the rest was overcome by drilling wells toward min where the induced fractures grow perpendicular to the wellbore.

The challenges associated with the drilling of wells toward min due to the high stress encountered by the borehole causing instability and hole breakout were overcome successfully by applying real time geomechanics (RTG) to assess reservoir pressure and predict fracture gradient during drilling.

“Based on this information, continuous adjustment to mud weight and fluid chemistry could be performed to achieve borehole uniformity and stability. Geosteering is also used to adjust well azimuth and inclination based of real-time responses to ensure that the wellbore stay within the reservoir,” adds Haas.

Among the technological advancements championed by Saudi Aramco include low gel loading, non-damaging fluids, and optimal chemical concentration.

It provides adequate fluid property to create and propagate a fracture, transport and pack proppant uniformly in the fracture ensuring high conductivity, and break the gel after the treatment is over for a fast and efficient cleanup, thereby increasing retained proppant permeability. Use of an improved chemical system for a second cleanup on some already fractured wells that were showing low productivity has revived them at a higher sustained rate.

Rigorous modelling with the best available data has enabled the engineers to design and select the optimal completion and stimulation practices to attain higher well productivity. The use of high strength proppant to encounter in stress has improved sustained fracture conductivity overcoming gel damage, fine migration, embedment, and proppant degradation with time. All of these factors, if not addressed properly, would adversely affect well performance.

High-strength and conductive proppants have also been the key factor to sustain rates. There are several factors that affect conductivity, such as non-Darcy effects, multi-phase flow, gel damage, constrictions, embedments, and fine migration. If all these factors are considered with non-optimised selection of fluids and proppants, then the actual fracture conductivity could be as low as 5% of the conductivity measured on the proppant pack in the laboratory.

“Saudi Aramco, through its many years of technical expertise and experience, has considerably improved in optimising both acid and proppant fracturing treatments and has successfully been able to keep the induced fractures open and conductive,” says Rahim.

Multi-stage completions include a fracturing string with packers and injection ports to be placed in the reservoir section. The number of ports selected depends on the reservoir development. Each port is isolated from the rest by open-hole packers.

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“The packers can be either mechanical or swellable,” Rahim tells Oil & Gas Middle East. “Mechanical packers are short in length, in the order of few inches, and are easy to run with the completion string. The swellable packers are long and can range from ten to thirty feet. Inclusion of long swellable packer and dual packer systems in the multistage fracturing (MSF) completions have proven successful.

“These packers assume actual hole shapes and deformations and cover hole breakouts and ovalities and providing sufficient insulation between adjacent fractures, thereby keeping them separate and independent from one another.”

This ensures high contact area of the wellbore with the reservoir, a very important milestone to achieve high production rate.

Application of RTG has practically eliminated drilling complications in the reservoir like excessive mud loss or break-ins and minimised drilling time and near well damage significantly ensuring smooth deployment of MSF assemblies. Application of MSF assemblies has led to successful well completion, fracturing and incremental sustainable gas production.

High-rate matrix acidising is a viable option to treat good reservoirs. Acid is pumped through multiple ports in a reservoir section that is isolated by packers. This technique etches the reservoir section uniformly and can also create short fractures. Aramco says the technology has “provided excellent results in moderate to high permeability wells”.

“Saudi Aramco continues to enhance the fracturing technology to ensure that the induced fractures are properly cleaned up after the treatment enabling high retained proppant conductivity,” says Al-Kannan.

“On the other hand, the fracture should maintain conductivity throughout the production life of a well so that well intervention can be avoided, and risk and cost optimised. To achieve this, new technologies have been evaluated, customised, and implemented.”

Depending on the reservoir properties, post-fracturing production may drop due to collapse of the fracture, particularly when high-drawdown pressures are applied to enhance production. In certain layered reservoirs, there can be poorly dissolvable anhydride streaks that can isolate part of the created fracture from the wellbore. One way to address these challenges is to pump proppant fracturing in a carbonate environment instead of the regular acid treatment.

“Proppant will permanently fill and support the created fracture and will prevent it from collapsing,” Rahim says.

“An additional benefit of proppant fracturing is significantly larger fracture half-length, as proppant covers most of the created fracture, while acid rapidly reacts with carbonate and cannot penetrate deeply into created fracture, particularly in high temperature environments.

Longer fracture lengths imply higher contact area, which contributes to improved gas rate. But conventional proppant fracturing in carbonate reservoirs may have some drawbacks, including: lower fracture conductivity; difficulty in achieving the required higher net pressure; and a high risk of premature screenout.”

Channel fracturing is an advanced proppant fracturing technique that can be used to mitigate the conventional proppant fracturing drawbacks. It substitutes the homogeneous proppant pack in the fracture by a heterogeneous structure containing a network of open channels.

“The channel conductivity values are a few orders of magnitude larger than conductivity of proppant pack in conventional fracturing,” Rahim says.

“The key difference between pumping schedules for the channel fracturing and conventional proppant fracturing is the addition of proppant in short pulses, separated by pulses of clean fluid. Discontinuity in proppant addition helps to reduce net pressure during treatment, staying below the completion limitation becomes easier,” he concludes.