Other effects .1 Healing

It has been observed that, with time, the strength of some formations after the initial reduction in strength caused by formation breakdown. In some cases the strength of the formation returns completely, in others only partially.

This process has been called "clay healing", because it only occurs in shales and not in carbonates. There are indications that it only occurs with water-based muds, and not with oil-based muds. The mechanism is not very well understood, and Opcos are invited to share their experiences with healing of formations.

The phenomenon can not be relied on, but justifies a repeat Leak-off test some time after formation breakdown has occurred.

The strength of a fractured borehole may be improved by squeezing cement in the fracture.

Theoretically, if the fracture is perfectly isolated from the wellbore, the original FBP will return.

This may never be achieved, even after several squeeze jobs. In addition, the strength of a borehole can never be improved to above the original FBP with cement squeezes.

4.6.4.2 Borehole fluid penetration

In the preceding sections it s been assumed that the borehole does not penetrate the formation.

When borehole fluid does invade the formation pore space, the near wellbore effective stresses will change as a consequence of pore fluid pressure modification near the wellbore. This phenomenon will reduce the strength of a wellbore. The magnitude of the reduction in strength depends on the quality of the mudcake, the permeability and the poro-elastic properties of the formation.

Most casing shoes are set in low permeability rock (shale) and the equations of section 3.2 apply. If more permeable formations are drilled, a mudcake will form on the borehole wall which will prevent further penetration of fluid into the formation. The pressure in the permeable formation will remain unchanged, and the formulas of section 3.2 will remain valid.

However, depending on the effectiveness of the mudcake, fluid may penetrate formations. This may be the case when clear fluids without fluid loss control are used (for example during work-over). If no mudcake is formed due to the low permeability of the formation ( for example in shales), borehole fluid may slowly enter the pore space and in time, the pore pressure will increase. This mechanism is thought to be responsible for time dependent shale instability problems. Because of capillary pressures, penetration of oil-based muds (OBM) is less than of water-based muds (WBM). This explains the better performance of OBM. Improved understanding of these phenomena is the subject of ongoing research.

4.6.4.3 Depletion

During reservoir depletion the in-situ stresses change. The total overburden stress will remain constant, which means that the effective vertical stress increases (see Eq. App. 3-1). The two horizontal stresses will reduce, and the effective horizontal stresses increase. This will reduce the formation strength. It may have additional consequences like the initiation of shear fractures, sand failure or compaction.

If we assume that the formation behaves in a linear poro-elastic manner, we can calculate the change in the horizontal stresses using :

∆σ2,3 = γ x ∆po (App. 3-10) According to the poro-elastic theory, the depletion constant γ can be expressed as follows:

γ =

β = ratio of rock grain compressibility to rock matrix compressibility, v = Poisson's ratio.

This depletion constant can be determined from in-situ stress measurements at different stages of reservoir depletion. For sedimentary basins, values of the depletion constant have been reported between 0.4 and 0.6 [10,11,12]. The reduction in formation strength caused by depletion can be calculated using the reduced minimum in-situ stress from Eq. App. 3-10. The same approach can be used to correct for the effects of inflation.

4.6.4.4 Borehole shape

The formulas for wellbore strength, given in section 3.2, have been derived for circular boreholes.

If the borehole is not round, the borehole possibly will fail at a lower pressure. No similar equations exist for out-of-shape boreholes (except for an elliptical Shape). For such cases, the use of numerical programs is required, for example STABOR [13]. For the purpose of casing design however, it can be assumed that the equations given in section 3-2 are still valid.

4.6.4.5 Chemical interaction

The chemical interaction between formation rock and the wellbore fluid (e.g. a sensitive shale and a water based mud) will also alter the conditions under which breakdown occurs. However, the mechanisms and parameters affecting those mechanisms are still under investigation [4,14,15].

4.7 References

[1] Jaeger, J.C. and Cook, N.G.W.

Fundamentals of rock mechanics Chapman and Hall, London, 1971 [2] Brady, B.H.G. and Brown, E.T.

Rock mechanics

George Allen & Unwin, London, 1985 [3] Veeken, C.A.M., KSEPL

Rock mechanics manual

SIPM/KSEPL (in preparation, issue date end 1993) [4] Bol, G.M., KSEPL

The interaction between shales and fluids, Parts I to V

EP 87-1171, EP 87-1451, EP 87-2672, EP 87-2748, EP 88-1563 [5] Mouchet, J.P. and Mitchell,A.

Abnormal pressures while drilling Elf, Boussens, 1989

[6] Fertl, W.H.

Abnormal formation pressures

Developments in Petroleum Science Engineering

Elsevier Scientific Publishing Company, Amsterdam, 1976 [7] Bourgoyne, A.T., Chenevert, M.E., Milheim, K.K. and Young, F.S.

Applied drilling engineering, Vol. 2

SPE textbook series, Richardson, Texas, 1986 [8] Eaton, B.A.

The equation for geopressure prediction from well logs SPE 5544,1975

[9] Wind, J.A. and Marchina, P , KSEPL

Formation strength for casing design Building Blocks for the Update of the 1980 Casing Design Manual

EP 92-1454

[10] Breckels, I.M., KSEPL

Relationship between horizontal total stress and depth in sedimentary basins, Part II Brunei Venezuela, the North Sea and the Netherlands

EP 05-4153

[11] Breckels, I.M. and van Eekelen, H.A.M., KSEPL

Relationship between horizontal stress and depth in sedimentary basins EP-52950

[12] Veeken, C.A.M., Hertog, G.M.M., Hydendaal, H.G.C. and van der Meulen, J.T., KSEPL Groningen sand failure study status report part 2: rock stress and rock strength in Groningen /Annerveen fields

EP 90-3389

[13] Wong, S.W. and Kenter, C.J., KSEPL

Borehole stability analysis part 1: theoretical formulation of STABOR RKRS 91.15, 59-64

[14] Hale, A.H. and Irani, F.K., BRC

DEA 22 report on the effects of drilling fluids on shale stability BRC 12.89

[15] Hale, A.H,, Irani, F.K. and Albrecht, M.E.S., BRC Hydration characteristics of shale, Parts I to IV BRC 43.87, 50.88, 13.89, 14.89

4.8 Appendix 4 : Procedures for leak-off and limit tests