Emerging Technology Brings New Life to “Old” Science
By Julie Anderson
Petroleum geomechanics is “the branch of engineering dealing with the mechanical behavior of geologic formations which influence the exploration, development, production, and storage of oil and gas,” as defined by GeoMechanics Technologies at http://www.geomechanicstech.com/geomechanics.html.
In more simplified terms, “geomechanics answers the question of how the geologic formations react against the disturbance we cause in earth by drilling, production, or other operations,” explained Vahid Serajian, geomechanics research engineer with GeoMechanics Technologies, with offices in Houston, Los Angeles, and Vienna.
Geomechanics is not a new field in and of itself, Serajian emphasized. As with many other aspects of the current boom, advances in technology have reinvigorated the use of this specialty and enabled it to be applied more fully across the industry. “Many of the analytical equations commonly used for quick analyses were developed decades ago,” Serajian said. “However, with development of the advanced numerical methods and powerful computers, geomechanics has reached a new milestone.”
The Way it Works
Perhaps the best way to familiarize ourselves with the seemingly unfamiliar term “geomechanics” is to ask a geomechanical engineer for a job description. Permian Basin Oil and Gas Magazine took a dual approach, first searching the “want ads,” and then asking Serajian, “What, exactly, do you do?” Interestingly, a quick online search yielded an impressive list of job opportunities related to geomechanics, with the following recurrent language:
- Deliver innovative technology to reduce drilling and completion risk and optimize reservoir production.
- Provide description of stresses and mechanical properties for a formation, field, or basin. Offer practical, cost-effective solutions to geomechanical problems.
- Support drilling operations through real-time geomechanics and wellbore stability studies for drilled wells.
- Apply geomechanics models to augment fracture stimulation; assist in drilling and well placement; minimize reservoir compaction, surface subsidence, and sand production.
- Provide real-time decisions to help drilling team to maximize efficiency and minimize risk while achieving the well objectives.
- Advise clients on drilling, stimulation, and sanding issues.
Generally speaking, the required job qualifiers included a bachelor’s degree in petroleum engineering, geology, or geophysics, and experience in one dimensional or three dimensional mechanical earth models.
At the University of Rochester in New York, students in the geomechanics program “are trained as earth engineers,” offered professor Cynthia Ebinger, editor-in-chief of Basin Research.
“Geomechanics in our program is a mix of basic science foundation courses (chemistry, physics, and calculus) with mechanical engineering, geology, and geophysics classes,” stated Ebinger, who works in the university’s Earth and Environmental Sciences Department. Serajian, a geomechanical engineer, described his job as follows:
Analyze core and log data to understand and estimate the mechanical properties of the geologic formations.
Run pore pressure prediction and analyze the data to estimate the in-situ stresses.
Use the geometry of the geologic formations, rock mechanical properties, reservoir pore pressure, and in-situ stresses to build numerical models and try to simulate the proposed operations to find out possible risks and to give advice on optimizing the operation, minimizing the failures, and avoiding high risks.
Work closely with other oil and gas professionals.
“We usually get our input data from geologists, reservoir engineers, and petrophysicists, and our results feed the data that drilling, production, and completion engineers require to optimally design a job,” Serajian said.
“In the oil and gas industry, in many cases the reaction of the earth against our disturbance in the in-situ stresses is ignored,” Serajian added.
Operations such as drilling, hydraulic fracturing, water/CO2 flooding, or oil and gas production disturb the initial in-situ stress state and usually generate induced stresses/strains in geologic formations, he continued. This induced stress and strain may cause unwanted phenomena such as bedding planes slippage (causing casing failures), surface subsidence (causing offshore platform losses and/or onshore surface deformations), and fault reactivation in geologic formations. “In many cases, catastrophic incidents such as blowouts, casing collapse, and wellbore instability can be predicted using geomechanical studies,” Serajian said.
To summarize, Serajian offered the following: “Geomechanics will help oil and gas managers, decision-makers, and technical staff come to a better understanding of the earth’s reaction to the drilling, stimulation, or production operations and help them optimize their designs and minimize the operational risks.”
The term is comprised of two terms, he observed: “geo” and “mechanics.” When the earth (geo) encounters a disturbance involving force and movement (mechanics), geomechanics comes into play.
Geomechanics: Industry Impact
Hamed Soroush, Ph.D., has been a geomechanics expert for some 18 years. In April 2013, Soroush, geomechanics director at Petrolern Ltd. in Houston, delivered a presentation to the Midland/Odessa Society of Petroleum Engineers titled “Non-conventional Geomechanics for Unconventional Resources.” He also authored an article for the Society of Petroleum Engineers’ publication, The Way Ahead, titled “Discover a Career in Geomechanics.”*
When it comes to the general science of geomechanics, Soroush offered Permian Basin Oil and Gas the following description: The knowledge of modeling rock behavior in response to any forces or pressure applied to it naturally (e.g. tectonic forces, underground fluid pressure, overburden load, etc.) or artificially (e.g. building structures such as high-rises, dams, and power plants on the rock; excavating shafts or tunnels; or drilling wells from a water or oil/gas reservoir.) Petroleum geomechanics covers the applications related to the oil and gas industry, only, Soroush continued. Geomechanical modeling requires good understanding of physics, mathematics, mechanics, and geology.
In his SPE article, Soroush indicated that while “the systematic application of rock mechanics in the oil and gas industry is relatively new, it was recognized and appreciated by many oil companies in a short period of time and has become a fast-growing field due to its applicability and effectiveness in reducing nonproductive time.”
Geomechanics was initially developed for mining and civil engineering, he said, but merged into the oil and gas industry in the 1980s to improve hydraulic fracturing and drilling operations. PBOG asked Soroush to explain how, specifically, geomechanics can impact the production side of the industry.
Rock formations below the water table are usually saturated by fluid—due to different forces and loads, this fluid has a hydrostatic pressure called pore pressure. Analyzing log data such as density, resistivity, wave velocity, or seismic data in conjunction with understanding the physics/mechanics of formations, faults, fractures, fluid, and temperature helps in the prediction of pore pressure, which consequently helps with drilling and production safety.
Cap rock integrity.
Usually, oil and gas reservoirs are trapped below an impermeable rock layer which is called cap rock. Before touching any reservoir, we should make sure that our prospective activities will not create fracture in the cap rock which can cause leakage or seepage of fluid out of the reservoir. Breakage of cap rock is tantamount to losing the oil/gas reservoir.
Field problem diagnosis.
Geomechanics can help in diagnosing and understanding any possible problems that might happen during drilling and production, and save millions of dollars for the oil and gas industry every year.
Formation properties evaluation.
The key to successful drilling, completion, and production operations is to have good knowledge of rock properties including porosity, permeability, stiffness, strength, and compressibility. This information can be achieved by performing rock mechanics testing in the lab and by analyzing well logs. Drilling, completion, and production plans can be designed using this information in conjunction with other data.
In-situ stress estimations.
Rock formations below the surface are under stresses from different sources including the weight of overburdened layers and tectonic forces. Unlike pore pressure, stress in rock is not isostatic, i.e. it varies in different orientations. Knowing the principal stresses in three perpendicular orientations (one vertical and two perpendicular horizontals), it’s possible to calculate stresses in any other orientation. These three main stresses are called vertical, minimum horizontal, and maximum horizontal in-situ stresses.
Drilling performance evaluation.
Building a geomechanics model for a field enables analyzing the drilling performance in previously drilled wells (postmortem analysis) in order to improve the performance for planned wells. It also enables predicting possible problems in future wells.
Geomechanics can provide guidance to drill a wellbore without any stability problems such as wellbore collapse, wellbore convergence, and fracturing the rock (which causes loss of drilling mud). This includes choosing the optimum drilling fluid pressure and composition, selecting optimum casing shoes, (wellbore geometry) and designing the best orientation to drill the well (wellbore trajectory). A wellbore stability study can save a significant amount of time and money during a drilling operation.
Borehole trajectory optimization.
This depends on which orientation and with which inclination a wellbore is drilled, as the stability condition changes. Geomechanics can provide us with the safest well trajectory for each geological and stress setup.
Sand production prediction and control.
Production from sandstone reservoirs, especially unconsolidated sandstone, is usually accompanied with production of sand grains which causes significant damages to the down-hole and surface facilities. Geomechanics can help to predict this kind of unwanted sand production and provide guidelines for completion and production design in order to prevent or control sand production. This increases the amount of produced oil and gas markedly.
Production maximization affected by natural fractures.
Natural fractures in reservoirs are the main source of production for oil and gas as they are usually more permeable and provide conduits for oil and gas flow to the well. However, without knowing the orientation of the fractures and also stress conditions in the reservoir, it is not possible to maximize production from natural fractures. Geomechanics can find the most permeable and productive fractures by conducting “critically stressed fracture” analyses and provide guidelines for the best trajectory to drill in the reservoir to maximize production. This is one of the greatest contributions of geomechanics in production from fractured reservoirs.
Hydraulic fracturing is one of the most important technologies in enhancing oil and gas production from reservoirs without enough permeability to produce economically without stimulation. It involves creating huge artificial fractures in the reservoir using hydraulic pressure. Geomechanics is the biggest contributor in designing and optimizing hydraulic fracturing operations.
The key to applying geomechanics to the impact areas listed above is the geomechanical model, Soroush stated. A geomechanic model is comprised of six main components including rock property, pore pressure, vertical stress, minimum and maximum horizontal stresses, and stress orientation.
“When a geomechanical model is combined with a reservoir and geology model of a field, it forms a three-dimensional geomechanical model,” Soroush specified.
In his SPE article, Soroush cited examples when geomechanical modeling “reduced the number of casings, resulting in significant cost savings for the operators.” He went on to address production in naturally fractured reservoirs, indicating that a geomechanical model can make a “real difference” by identifying critically stressed factures which are actually productive fractures, thus optimizing drilling orientation.
Geomechanical models can be applied to specific well designs to assess performance and damage risks under varying deformation conditions, according to GeoMechanics Technologies, which develops geomechanical models from the reservoir scale to the individual well component scale to predict subsidence and well damage risks and to develop mitigation strategies to reduce such risks.
According to these geomechanical professionals, the seemingly unfamiliar science of geomechanics has promising possibilities when it comes to the petroleum industry, so much so that Soroush described geomechanics as “the oil and gas industry’s missing link.”
Editor’s Note: This article includes information from the The Way Ahead, a publication of the Society of Petroleum Engineers (SPE). The article, “Discover a Career in Geomechanics,” ran in the Vol. 9., No. 3, 2013 issue, http://www.spe.org.twa/print/archives/2013/2013v9n3. Copyright © 2013 SPE.
Julie Anderson, based in Odessa, is editor of County Progress Magazine, and is well known to many readers of PBOG as the former editor of this magazine.