An improved procedure for pre-drill calculation of fracture pressure

Abstract Pre-drill modelling of fracture pressure (FP) is an essential part of well planning, reserve estimation and evaluation of the potential for inducing seismicity as the result of fluid injection. Estimation of stress ratio or Poisson's ratio values or compaction state with depth is required in frequently used FP models. A new method to estimate FP is proposed which is based on Leak Off (LOT) and pore fluid pressure (Pp) data from offset wells and vertical stress (Sv)–depth relationships. LOT/Sv ratios observed in intervals of offset wells that are normally pressured (hydrostatic) are used to define an expected FP/Sv ratio for hydrostatic Pp conditions for all depths. Typical FP/Sv ratios for hydrostatic conditions derived using LOT data range from 0.81 to 0.89. Observed LOT values associated with Pp greater than hydrostatic (overpressured) in offset wells are used to quantify the rate of increase in FP with increasing overpressure (OP). The expected FP for hydrostatic conditions is compared with observed LOT values from depths where the pore fluid is overpressured and a relationship of increased FP, relative to the expected FP for hydrostatic conditions (residual FP (FPr)) with increasing OP is defined. The FPr:OP ratio typically ranges from 0.24 to 0.43. Fracture pressure models developed by this procedure may be used to predict FP for wells in different water depths and with Pp conditions different from those in the offset wells. The use of the model is demonstrated in three case studies taken from different geological settings: the Scotian shelf (offshore Nova Scotia), offshore Central Gulf of Mexico and the chalk interval from the Central North Sea.

[1]  R. Swarbrick,et al.  Estimating pore fluid pressure-stress coupling , 2016 .

[2]  P. Flemings,et al.  Reservoir pressure and seafloor venting: Predicting trap integrity in a Gulf of Mexico deepwater turbidite minibasin , 2005 .

[3]  Richard E. Swarbrick,et al.  Present-day stress orientation in Brunei: a snapshot of ‘prograding tectonics’ in a Tertiary delta , 2005, Journal of the Geological Society.

[4]  R. Hillis,et al.  Subsurface Sediment Mobilization , 2003 .

[5]  N. Dutta.  Deepwater geohazard prediction using prestack inversion of large offset P-wave data and rock model , 2002 .

[6]  D. Grauls,et al.  Overpressures: Causal Mechanisms, Conventional and Hydromechanical Approaches , 1999 .

[7]  T. Engelder,et al.  Influence of poroelastic behavior on the magnitude of minimum horizontal stress, Sh in overpressured parts of sedimentary basins , 1994 .

[8]  I. M. Breckels,et al.  Relationship between horizontal stress and depth in sedimentary basins , 1982 .

[9]  Ben A. Eaton,et al.  Fracture Gradient Prediction and Its Application in Oilfield Operations , 1969 .

[10]  R. Swarbrick,et al.  Role of the Chalk in development of deep overpressure in the Central North Sea , 2010 .

[11]  J. Nelson,et al.  Northern Cordilleran terranes and their interactions through time , 2007 .

[12]  R. Davies,et al.  Birth of a mud volcano : East Java , 29 May 2006 , 2007 .

[13]  R. Hillis.  Pore pressure/stress coupling and its implications for rock failure , 2003, Geological Society, London, Special Publications.

[14]  M. Nashaat.  Memoir 70, Chapter 10: Abnormally High Formation Pressure and Seal Impacts on Hydrocarbon Accumulations in the Nile Delta and North Sinai Basins, Egypt , 1998 .

[15]  L. Gaarenstroom,et al.  Overpressures in the Central North Sea: implications for trap integrity and drilling safety , 1993 .

[16]  J. S. Bell,et al.  Lecture: The Stress Regime of the Scotian Shelf Offshore Eastern Canada to 6 Kilometers Depth And Implications For Rock Mechanics And Hydrocarbon Migration , 1989 .