Perforation Cavity Research Articles

Flow geometry effects on sand production from an oil producing perforation cavity

Abstract Physical model experiments have been performed on perforation cavities in a weak sandstone. The objective of the study has been to investigate the effect of the flow geometry on sand production risk. Two extreme geometries have been tested, one with flow mainly axially towards the tip of the cavity and one with fluid flowing radially towards the side of the cavity. The experiments have been performed in a special pressure vessel for sand production studies. We have recorded the deformation of the cavity, the flow rate through the sample and monitored sand production and acoustic emission activity as functions of the imposed confining pressure and pore pressure drop across the sample. From the tests, we have found initial cavity failure and initial sand production at various combination of confining and pore pressure. The results have also been corroborated by finite element simulations. We have concluded that by directing the flow through the bottom of the cavity sand production is inhibited compared to the case where flow mainly enters through the side of the cavity. This is because the cavity fails along the wall first, while the bottom is intact. As flow promotes the removal of failed material, radial flow increases the risk of sand production.

Flow geometry effects on sand production from an oil producing perforation cavity

Flow geometry effects on sand production from an oil producing perforation cavity

Modelling Of Cavity Stability And Sand Production In Heavy Oil Reservoirs

Abstract Cold flow production of unconsolidated bituminous reservoirs reduces the in situ effective stress state to a very low value. The unloading coupled with viscous gradient leads to the yielding and progressive failure of the perforation cavity. The stability of the cavity is primarily influenced by the pressure gradient at the cavity boundary and the cohesion of oil sands. In this paper, a three-dimensional cavity stability criterion is derived based on the equilibrium considerations at the cavity boundary. It is found that a perforation cavity will remain stable even with substantial yielding so long as the cohesive strength remains intact. Local collapse and cavity propagation occur when the critical seepage force is achieved. The direction of cavity growth is therefore a function of the local flow regime, fluid and rock properties, boundary conditions and cavity geometries. For a longitudinal perforation cavity, streamlines converge to the tip where the exit pressure gradient is the largest. High-resolution simulation of near-cavity flow indicates that substantial pressure gradient can be developed in the transient period to cause cavity collapse. Based on the comparisons of simulation results and the cavity stability criterion, the possible mechanisms for cavity growth under field conditions are evaluated. Introduction The primary production of heavy oil is emerging as a viable recovery technique. This method involves the solution gas drive mechanisms as well as sand co-production. The solution gas drive and foamy oil mechanisms have been discussed by a number of recent papers(l-3). Field production data appear to suggest that these production mechanisms are rather complex and are quite different from those encountered in the conventional solution gas drive process in structurally rigid reservoirs. One of the unique features is that a good oil production rate is generally accompanied by sand production. Sand production is believed to increase the effective area of inflow near the wellbore. One of the possible scenarios for the enhanced permeability zone is the formation of long horizontal channels. Wong et al.(4) found that a circular channel of less than 150 mm in diameter in oil sands retains its integrity even under an overburden effective stress of 6 MPa, provided that there is no inflow into the cavity. The seepage force as a result of the exit pressure gradient is the key factor causing the instability of sand cavity and subsequent sand production. In this work, the investigation of Wong et al. is extended to consider the stability of a general three-dimensional cavity. Using an equilibrium stability analysis, together with the Mohr-Coulomb failure criterion, and the kinematic condition for shear stress mobilization, the critical condition of cavity stability under the influence of seepage forces is derived. This condition is cast in the form of a dimensionless stability number. The influence of exit pressure gradient on cavity stability is investigated using a decoupled approach where the flow around the cavity is modelled with high-resolution flexible-grid reservoir simulation utilizing the recently developed control-volume finite-element (CVFE) technique. In this work, the simulation is restricted to single phase.

Experimental study of sand production from perforation cavities : J. Tronvoll & E. Fjaer, International Journal of Rock Mechanics & Mining Sciences, 31(5), 1994, pp 393–410

Experimental study of sand production from perforation cavities : J. Tronvoll & E. Fjaer, International Journal of Rock Mechanics & Mining Sciences, 31(5), 1994, pp 393–410

Experimental study of sand production from perforation cavities

Sand production from perforation cavities has been studied by means of physical model experiments of an in situ perforation cavity on a 1:1 scale. The study focused on the mechanisms of cavity instability, sand production and, in particular, the effect of fluid flow. The test samples were cylindrically shaped with a coaxial cylindrical cavity. The samples were loaded isotropically at the outer surface with fluid inflow only through the non-perforated end surface. The evolution of a failure zone around the cavity was significantly affected by the fluid flow. The stress at initial failure of the cavity wall appears not to be significantly affected by the fluid flow.

Permeability reduction due to grain crushing around a perforation

The permeability reduction around a perforation cavity is analyzed as the result of grain crushing due to the perforation process. The poroelastic solution of a plane wave emanating from a cylindrical cavity is used to derive expressions for the intergranular stresses responsible for grain-crushing phenomena. These phenomena are examined on the basis of Hertzian contact theory of fracture and micromechanical descriptions of granular assemblies assisted by discrete element simulations. Results are presented for hydrostatic loading conditions for comparison with existing experimental results, and for uniaxial strain conditions as an approximation to the stress field around the perforation. The analysis can be applied to the prediction of the extent of the permeability-reduced zone and used for estimating the permeability reduction around the perforation.

The effect of anisotropic stress state on the stability of perforation cavities

The stability of perforation cavities is a key issue to minimize sand influx into the wellbore during production of hydrocarbons.

Experimental investigation of perforation cavity stability : Tronvoll, J Proc 33rd US Symposium on Rock Mechanics, Santa Fe, 3–5 June 1992P365–373. Publ Rotterdam: A A Balkema, 1992

Experimental investigation of perforation cavity stability : Tronvoll, J Proc 33rd US Symposium on Rock Mechanics, Santa Fe, 3–5 June 1992P365–373. Publ Rotterdam: A A Balkema, 1992