Deformation and yield in high porosity outcrop chalk

Deformation and Yield in High Porosity Outcrop Chalk
R. Risnes
Stavanger University College, School of Science and Technology, P. O. Box 2557 Ullandhaug, N-4091 Stavanger, Norway Received 13 June 2000; accepted 25 September 2000 Mechanically high porosity chalks behave as frictional materials, but with an end-cap reflecting pore collapse failure. Shear failure between the grains seems to be the basic failure mechanism, not only in compression tests at low confining pressures where shear bands are formed, but also in pore collapse where distributed shear failure will occur in the material. Also tensile failure seems to be initiated by a shear failure mechanism, and tensile strength can be predicted from the compressive yield criterion (Mohr-Coulomb). The mechanical properties of chalk are strongly dependent on the type of fluid in the pores. Water saturated chalk is considerably weaker than dry or oil saturated chalk. Chalk and fluids may interact through capillary forces and through surface physical/chemical reactions. Capillary forces will always be present when two immiscible fluids are present in the pore space. But such effects can only partly explain the experimental observations. There are however indications that the water weakening effect might be caused by repulsive forces generated by dipole-dipole interactions in the very narrow grain contact areas. © 2001 Elsevier Science Ltd. All rights reserved.
Pergamon
Phys. Chem. Earth (A), Vol. 26, No. 1-2, pp. 53-57, 2001 © 2001 Elsevier Science Ltd All rights reserved 1464-189510115 - see front matter PII: S 1464-1895(01)00022-9
1 Introduction High porosity chalks are important reservoir rocks in the southern part of the North Sea. The most important examples in the Norwegian sector are the Ekofisk field (Phillips) and the Valhall field (BP Amoco). Most of the oil and gas fields in the Danish sector are also chalk type reservoirs. Many of lhe chalk reservoirs have preserved high values o f porosity, sometimes exceeding 40 %. Early invasion of hydrocarbons together with formation o f overpressures, have prevented normal compaction, and limited the diagenetic processes. The reservoir chalk formations appear on the surface many places in Europe. This offers the possibility of finding outcrop chalk with properties similar to the reservoir chalks. As core material from chalk wells necessarily is rather limited, outcrop chalk represent an interesting substitute, available in large quantities. Apart from being an important reservoir rock, chalk is by itself a very fascinating granular material. Pure chalks are Correspondence to: R. Risnes 53
Abstract.
mainly built up of whole and fragmentary parts of calcite skeletons produced by planktonic algae. The building blocks of the skeletons are calcite tablets or platelets of typical dimension 1 ~tm. These calcite grains are arranged in rings or rosettes known as coccoliths, typically of the order of 10 ~rn in diameter. Pure high porosity chalks consist of a mixture of intact coccolith rings and greater and smaller fragments. This gives the chalk material a rather open structure, where the dimensions of the pore space may be considerably greater than the dimensions of the individual grains. Porosity may be greater than 40 %. On the other hand, due to the smallness of the grains, the pore throats will be narrow, and the permeability of such chalks will be rather small. A picture of a typical high porosity chalk is presented in Fig. 1.
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R. Risnes: Dh Porosity Outcrop Chalk
into the material (Risnes et al 1994). The spontaneous water imbibition compresses and expels the air in the samples, and the corresponding overpressure is probably the main driving force in the dissolution process. Solid-solid contacts may be formed through the asperities in the narrow contact regions between the grains. In fact, some experiments where the chalk has been highly compressed indicate the presence of such a sintering effect (Risnes et al 1998). Other grain contacts may be purely frictional in the sense that the grains are not bonded together. But due to irregularities at the grain surfaces, and the complex structure of the chalk assembly, also such contacts may contribute to the cohesion of the material. The chalk material may also be reinforced (or weakened) by greater calcite remains from other organisms deposited at the same time as the coccolith rings. Development of silica crystals will normally increase the strength of the chalk. Compared to other rocks, chalk is a rather special material in the sense that their main constituents, the coccolith rings, are generated within living ceils. How the grains making up the rings, are linked together is not known, but obviously the rings will have an intrinsic stiffness that will contribute to the strength of the chalk. Another important aspect is the grain surfaces which may be very different from those o f calcite crystals grown from anorganic solutions. It has also been demonstrated by using different solvents (Baldwin 1985) that in some cases the chalk grains will have an organic coating, maybe a reminder o f how the grains originally were formed. Such coatings may only be some monolayers thick, and thus not directly observable, even in an electron microscope. It is therefore not unexpected that chalks may exhibit a rather complex mechanical behaviour.