Compressibility of deep fluids-filled cavern is discussed. Compressibility is measured both through statical and dynamical tests. Statical compressibility is influenced by cavern shape and cavern fluids nature. This parameter plays an important role for such applications as the determination of stored hydrocarbons volume, of volume lost during a blow-out, and of pressure build-up rate in a closed cavern. Dynamical compressibility is measured through the periods of waves triggered by pressure changes. Both tube waves and longer period waves associated to the existence of an interface between a liquid and a gas can be observed. They can provide additional information, for instance the existence of trapped gas in the well-head.
This study presents simple graphical methods (0 predict itt-situ formation compressibility from production data. The graphical methods are based on the conventional material balance equation for oil reservoirs. The material balance equation is recasted into several straightline formats such that either the intercept or the slope of the straightline yields an estimate of formation compressibility. Also, floe information from the straightline plots can be used to predict oil-in-place.
The new predictive methods were applied to field data from a depletion/compaction drive reservoir. The formation compressibility and oil-in-place values predicted by the proposed methods were compared with those from other sources and methods, resulting in excellent agreement.
The proposed graphical techniques are limited to depletion/compaction drive reservoirs and can not be applied to reservoirs subject to strong water-drive.
Volumetric strains during triaxial tests of 44 sandstone materials were analysed in order to understand the physics of dilation. The samples originate from different North-Sea fields, and the porosities vary from 1 to 39 percent. Different confining stresses were applied to the same material under dry and saturated conditions and the results analysed. The results showed no clear boundary between dilation dominated and compaction dominated behaviours with respect to porosity or confining stress. However, two regimes could be identified: one with low porosities where dilation occur, and one with high porosities where dilation do not occur. The critical porosity separating the two regimes was found to depend on the confining stress. The analysis also showed that the shear stress level at the onset of dilation depends on the ratio between the in situ horizontal stress under which the sample was buried, and the confining stress. A mathematical formula relating the onset of dilation to porosity, confining stress and in situ horizontal stress has been established. The dependency of in situ horizontal stress is likely to be ascribed to core damage. This assumption is supported by previous experimental work on artificial sandstone. Volumetric strain analysis showed that fully saturated samples dilate more than corresponding dry materials. The onset of dilatancy for both saturated and dry materials can be described by the same mathematical formula, however with different coefficients. The results from the use of the model showed that the onset of dilation could be estimated with a fair accuracy if the porosity and the in situ horizontal stress are known.
Wear development during bit life modify the mechanical response of PDC bits and reduce their efficiency. Contrary to cutting mechanism, friction mechanism on wear flats which develops on the cutters, remains badly understood. A new friction law based on numerical and experimental studies has been stated by the authors. It depends on the material behavior, and relates the applied normal pressure to the wear flat to the subsequent dip angle a into the rock surface. Drilling parameters for PDC bits can then be evaluated, provided a reasonable wear law be postulated. Two different experimental benches have been built up, one for determining the parameters of cutting and friction mechanisms on a single worn cutter, and the other one for determining the global parameters on a small scale drill bench. Preliminary data allow to discuss some hypotheses of the model.
This paper presents a field case based upon a reservoir operated by Statoil in the Norwegian Sea. The case concerns a series of water injectors - i.e. both sub-sea and platform - that underwent extreme losses of injectivity over short periods of time. When worked over, the wells showed extreme amounts of sand fill that sometimes were several hundreds of meters above the top perforation. Furthermore, the link between well shut-ins and injectivity losses was clear right from the onset of the study.
The life of the injectors is thoroughly reviewed and the reasons for the injectivity losses are established.
1.First, it is shown that even under no flow conditions corresponding to shut in periods, the rock around the wells is too weak to sustain the stresses and fails.
2.Second, it is established that because of permeability heterogeneity, the wells are cross flowing during shut in periods hence allowing sand to be produced in front of the perforated interval.
3.Third it is shown that under routine operation conditions, the produced particles in front of the perforated intervals are not able to settle down in the rat-hole before injection restarts and hence plug the perforation tunnel upon injection restart.
4.Finally, it is demonstrated that during a standard shut in, pressure waves as large as 80 bars are generated because of the so-called water hammer effect that hits the formations as would a seismic wave do. As a consequence, the formation already weakened by sand production undergoes liquefaction that triggers large amounts of sand to be released in the well, hence killing totally its injectivity.
Finally, the paper presents how the operating conditions of the wells were successfully changed to avoid the repetition of the problems experienced previously.
Couvreur, J.F. (Universite Catholique de Louvain) | Thimus, J.F. (Universite Catholique de Louvain) | Lousberg, E. (Universite Catholique de Louvain) | Vervoort, A. (Katholieke Universiteit Leuvan) | King, M.S. (Imperial College London)
A number of laboratory tests (uniaxial, triaxial and hydrostatic) were conducted on a dry porous limestone. A conceptual model is proposed to correlate deformation and damage fields with the variation of ultrasonic velocities and quality factors. This correlation is presented in a stress deviator versus confining pressure diagram. In this way, the successive steps occurring in the damage process of this rock are well described. In particular, the quality factor of the S wave distinguishes clearly the onset of the initially stable cracking, while velocity of the S wave and strain measurements are sensitive to dilatancy which appears later in the damage process of the studied limestone.
Compressional wave anisotropy caused by a single fracture and three parallel fractures was measured using an acoustic imaging system. The measured compressional wave transmission coefficients are compared with theoretically predicted transmission coefficients from the displacement discontinuity theory. Deviations from the displacement discontinuity theory occur at glancing angles of incidence to the fracture. The anisotropy in the transmission coefficient is observed to be frequency dependent and results in an apparent frequency-dependent stiffness.
In this paper the effects of borehole stress concentrations on both dipole flexural waves and azimuthally resolved, axially propagating compressional waves (i.e., microsonic) are studied. Dipole sonic logging tools provide the capability to measure acoustic anisotropy in the borehole. However, one needs to distinguish between two forms of anisotropy: intrinsic or stress-induced. A method is presented, based on an analysis of dipole dispersion curves, which can make this distinction. Once stress-induced anisotropy is determined, then the fast shear direction is the maximum stress direction. In addition, an azimuthal microsonic measurement technique is used to measure the azimuthally resolved, axially propagating, refracted compressional wave which can identify both stress direction and detect mechanical damage (i.e., zone of plasticity). Laboratory borehole experiments are presented for both dipole and compressional measurements. The experiments compare favorably with the theory of acoustoelasticity. The maximum stress direction is important in hydraulic fracturing while detection of mechanical damage can be important for wellbore stability.
This paper was prepared for presentation at the SPE/ISRM Eurock '98 held in Tronheim, Norway, 8-10 July 1998.
The principal function of leak-off tests is to assess casing integrity, but the test data are commonly used beyond this original purpose, for stress estimations in exploration and drilling. The value of leak-off test data for stress estimation is questionable and extended leak-off tests have been proposed as a more appropriate method for stress estimation. The paper for the first time compares leak-off test (LOT) and extended leak- off test (ELOT) data from two areas, the North West Self of Australia and the Norwegian North Sea. Standard LOTs have a characteristically large scatter associated with the leak-off pressures (Plo), minimum stress and fracture gradient estimates in both areas. However, the minimum stress determined from ELOTs are consistent and form a lower bound to the standard fl0 data in both areas. The ELOT takes about an hour to perform, but provides far superior data than that obtained from a LOT, and is recommended where stress data are required.