A New Simple Method To Estimate Fracture Pressure Gradient (includes associated paper 37685)
- L.A. Rocha (Petrobras S.A.) | A.T. Bourgoyne (Louisiana State U.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- September 1996
- Document Type
- Journal Paper
- 153 - 159
- 1996. Society of Petroleum Engineers
- 5.1.1 Exploration, Development, Structural Geology, 4.1.2 Separation and Treating, 5.6.1 Open hole/cased hole log analysis, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.6 Drilling Operations, 1.7 Pressure Management, 4.1.5 Processing Equipment
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Projecting safer and more economic wells calls for estimating correctly the fracture pressure gradient. On the other hand, a poor prediction of the fracture pressure gradient may lead to serious accidents such as lost circulation followed by a kick. Although these kind of accidents can occur in any phase of the well, drilling shallow formations can offer additional dangerous due to shallow gas kicks, because they have the potential of becoming a shallow gas blowout leading sometimes to the formation of craters.
Often, one of the main problem when estimating the fracture pressure gradient is the lack of data. In fact, drilling engineers generally face situations where only leak off test data (frequently having questionable results) are available. This problem is normally the case when drilling shallow formations where very few information are collected.
This paper presents a new method to estimate fracture pressure gradient. The proposed method has the advantage of (a) using only the knowledge of leak off test data and (b) being independent of the pore pressure. The method is based on a new concept called "pseudo-over- burden pressure", defined as the overburden pressure a formation would exhibit if it were plastic. The method was applied in several areas of the world such as U.S. Gulf Coast (Mississippi Canyon and Green Canyon) with very good results.
Overburden pressure gradient is defined as the pressure variation by depth due to the weight of the rocks' matrix and fluids in the rock pore spaces. If the bulk density (Pb) is a known function of depth, the overburden pressure for each depth can easily be calculated by integrating bulk density function versus depth. Pore pressure gradient is the pressure gradient of the fluid contained in rock pore space. The fracture pressure gradient is defined as the pressure gradient that will cause fracture of the formation. In other words, if a formation is exposed to a pressure higher than its fracture pressure limit, the formation will fracture and a loss of circulation will occur. Extreme problems related to loss of circulation can vary from well collapse (due to the decrease in hydrostatic pressure), to a quite severe gas kick (also due to the decrease in hydrostatic pressure) followed by a underground blowout. The consequences of an underground blowout are unpredictable. In the best scenario, the formation fluid will stay confined underground; however, it may migrate toward shallow and unconsolidated sediments resulting in a crater. Collectively, these aspects make formation fracture pressure knowledge fundamental when drilling oil wells.
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