Q: How do I know my well is positioned correctly with respect to the maximum horizontal stress?
A: In a recent paper by Petro.ai Science Advisor, Dr. Mark Zoback and coauthor, Dr. Jens-Erik Lund Snee both of Stanford University, State of stress in areas of active unconventional oil and gas development in North America, Zoback and Snee “present new data sets to help improve operational efficiency by constraining absolute stress magnitudes and the ideal azimuth to drill horizontal wells, i.e., perpendicular to the local SHmax orientation and make it possible to predict which fractures and faults are likely to be activated during hydraulic stimulation.”
“Here Zoback and Snee,” Dr. Brendon Hall, VP of Geoscience explains, “look at the direction of maximum horizontal stress across the US., compiling basin by basin hundreds of new measured orientations of SHmax. Using the measured maximum horizontal stress directions, you can determine the optimal way to drill your well following those stress principles.
“You know you want to drill in the earth perpendicular to the direction of the maximum horizontal stress. In the earth there are three principal stresses: Vertical stress which is the weight of the earth on top of where you are and the other two stresses are on a horizontal plane. These horizontal forces are perpendicular with respect to each other and are the minimum and maximum horizontal stress in a normal or a strike slip faulting area which is typically where we are in oil and gas plays.
“You want your fracs ideally to be set up geometrically. You want to be drilling in a direction perpendicular to the maximum horizontal stress so that when you frac those wells they have the easiest pathway to create a fracture network which opens up the stimulated rock area to develop the most efficiently producing horizontal wells.
“Zoback has spent a lot of his career measuring and quantifying the stress state in the US. The measurements of the horizontal stress are often done by looking at well bore breakouts. When an operator is drilling a well, if it isn’t controlled in the right way, say they have mud in there that’s too heavy, the well can start to deform in various ways. If it starts to fail because the mud is too light, it will start crumbling in the well bore at the point where the maximum horizontal stress is the greatest. At that point the well will compress and breakout. The direction of those breakouts will give you the direction of the maximum horizontal stress.
“As for the importance of drilling in this orthogonal direction of maximum horizontal stress, Zoback also published a recent study in the Bakken showing an area with similar rock quality but with wells drilled in many different orientations. He demonstrated that the direction of the wells does have a significant impact on production, showing that wells oriented correctly produce more barrels per foot than wells in other directions with “a significant economic uplift of several million dollars per well.”
“So how do you know your well is positioned correctly? Petro.ai handles all of that for you. When we build our machine learning models we are always calculating the direction of the well with respect to the maximum horizontal stress. As a feature we automatically extract and have a copy of Zoback’s world stress map that is up to date and in the data base.”
