Hassi Messaoud: Take Home Lessons on How Geomechanics Impacts a Giant Oil Field

Speaker: Dr. Lonnie Kennedy

Abstract: 

Geomechanics rose from obscurity in the last decade to help understand hydraulic fracturing behavior, especially related to horizontal multistage horizontal wells.  Unfortunately, for U.S. oil fields there is surprisingly little data to ground truth our assumptions on geomechanics. This is not true for the oil field of Hassi Messaoud (HMD), Algeria.  This giant is estimated to have over 25 billion barrels of oil.   HMD has over 800 vertical wells and as many horizontals.  Most vertical wells were full hole cored through the entire reservoir thickness (~ 450 ft), laboratory analyzed in detail, logged with every petrophysical tool and subjected to direct fracture injection testing (DFIT).

HMD has remarkably homogenous, layer-cake deposition, ranging from coarse grained alluvial fans to fine grained near-shore marine sands, in ascending stratigraphic order.  However, wells with similar lithology show starkly different permeability, ranging from 1,000 to 0.1 mD over short distances, often less than a mile.  Accordingly, ultimate reserves range from tens of millions to just tens of thousands of bo/well.  Why?

This research shows that tectonic stress is the dominant factor influencing permeability and oil well productivity at HMD.  A series of NE-SW pervasive faults exist at HMD, most likely formed from paleo-stress oriented about N-S.  The current stress system is strike-slip, NW-SE, and oriented 90o to the fault system.  This stress causes secondary buckling across the field perpendicular to the major horizontal stress (σH).  In some areas, the paleo-fault system has reactivated.  

In folded/faulted regions the rock has “failed” impeding its ability to support strain under regional tectonic stress. From DFIT results minimum horizontal stress (σh) is low along the crest of secondary buckling and in the proximity of reactivated faults.  In one folded/faulted region, σh is equal to the original reservoir pore pressure (6,730 psi).  The reservoir is over pressured by massive artesian water drive. As Pp≈σh indicates that the formation was likely undergoing natural auto-hydraulic fracturing.  In other regions, σh is nearly 15,000 psi exceeding our ability to initiate hydraulic fracturing.  These super high-stress regions are oriented at the poles of the HMD anticline where stresses converge.  DFIT stress does not correlate to the typical Eaton’s equation used industry wide to estimate σh but empirical equations are reasonable.  

Under stress, silica sands undergo secondary chemical silicification.  Grain contacts dissolve and reprecipitate in the pore throats.  Thus, the original permeability is destroyed in high stress areas, preserved in low stress areas and greatly improved where low stress and auto-fracking occurred.  Bands of high or low permeability develop perpendicular to σH. This explains the highly variable oil productivity of HMD field and the anisotropy observed.  Lessons in HMD can be applied to certain fields in the USA.