On the variability of fracture surfaces in unsaturated chalk (original) (raw)

Impact of intermittent rainwater and wastewater flow on coated and uncoated fractures in chalk

Water Resources Research, 1999

Two coated and two uncoated slices from the fracture surface of an unsaturated chalk were exposed to short flow events (24, 8, and 9 hours) of industrial wastewater and/or synthetic rainwater, followed by long drying periods (weeks). The topography of the fracture surface was shown to be unstable due to the detachment of colloidal and large-sized particles during the first 3-7 hours of flow. Following rainwater flow, erosion was more pronounced on the coated than on the uncoated surface (mean erosion of 0.313 and 0.134 mm, respectively). Interaction with industrial wastewater generated a skin of organic matter and gypsum that collapsed following contact with rainwater, leading to a deeper erosion of the uncoated surface (1.238 mm) than of the coated one (0.549 mm). Erosion was measured using a laser-scanning system and was calculated from high-resolution topographical maps (elevation z Յ Ϯ 0.01 mm) generated by Geographic Information System (GIS, ARCInfo) prior to and following the flow experiments. The mean thickness of the erosion was found to be strongly correlated with the thickness of a layer calculated from the total accumulated mass of particles and soluble salts released from the fracture surface. This relationship can be used to evaluate fracture surface erosion in large field and laboratory experiments.

The Effect of Fluid Content on the Mechanical Behaviour of Fractures in Chalk

Rock Mechanics and Rock Engineering, 2000

¶The paper presents an experimental study on the effects of fluid content on the mechanical behaviour of natural fractures in chalk. The aims of the study are to provide better understanding of the mechanisms of chalk-fluid interaction, in general, and to explain the behaviour of petroleum chalk reservoirs during water injection, in particular. The experiments were carried out on Lägerdorf chalk using the direct shear apparatus. Two types of fluids were used in the tests: 1) water, and 2) synthetic oil. Lägerdorf chalk is a water-wet material which will develop capillary pressures upon contact with water. Initially saturating the chalk with oil will enhance the water wettability by inducing additional capillary forces between water and the non-wetting oil. In addition to the tests on fractured chalk samples, unconfined compression and direct shear tests on intact chalk samples were performed. The results showed significant differences in the strength and deformation characteristics of intact chalk initially saturated with different fluids. Intact water-saturated chalk showed lower deformation modulus (about 50%) and lower peak (also about 50%) and residual shear strength than the oil-saturated chalk. Water injection in initially oil-saturated fractures resulted in significant normal deformation under constant effective normal stress and shear stress relaxation under fixed shear displacement. The water-induced deformation occurred almost instantaneously after only a few cm3 of water had been injected into the fracture, and further injection of water did not increase the water-induced deformation. After water injection, fractures in initially oil-saturated chalk showed significantly lower normal and shear stiffnesses and lower shear strength. The weakening in shear is attributed partly to the reduction in the basic friction angle, φb, and this reduction was verified in a series of tilt tests to measure the frictional resistance between smooth edges of core samples of chalk. The reduction in the basic friction angle implies that the interaction of chalk with water is governed not only by capillary forces, as postulated in several previous studies, but also by chemical and/or physio-chemical effects.

Micro- and nano-analyses of fracture-filling after flooding on-shore chalk with different IOR fluids

2018

Water injection into the Ekofisk-reservoir was introduced in 1987 to enhance oil recovery (EOR) dissolution and precipitation by exposing chalk to various brines at reservoir conditions, which further increased deformation of chalk matrix. This deformation affected mechanical properties of the tested samples and is referred to as water weakening of chalk. This thesis has focused on identifying mineral changes and mapping the distribution of precipitated mineral during flooding of samples which have an artificial fracture and will compare hollow cylinder to intact chalk cores. Methods used to achieve a mineralogic map are:

Chemical Effects on Chalk Weakning and Fracture Deformation

IOR 2009 - 15th European Symposium on Improved Oil Recovery, 2009

Steam injection is becoming increasingly used to enhance heavy oil production even in complex reservoirs such as fractured carbonates. However, injecting steam into fractures has the potential to change reservoir permeability because increasing the temperature causes the reservoir rock to expand potentially closing fractures and condensed water may react with the reservoir rock; both processes may increase uncertainty in predicting oil recovery from these reservoirs.

Chalk fracture system characteristics: implications for flow and solute transport

Quarterly Journal of Engineering Geology and Hydrogeology, 1995

Bulk groundwater flow and solute advection occur in the fractures of the UK Chalk, and a knowledge of the frequency and aperture of these fractures is crucial to understanding these two key processes in the Chalk. Fracture frequencies in the Chalk of southeast England have been assessed regionally by means of outcrop scanline measurements. Measurements of radon dissolved in Chalk groundwaters allow the estimation of fracture apertures in the saturated zone. These data provide valuable controls on the conceptualization and estimation of hydraulic properties for the Chalk aquifer. In particular, standard hydraulic models of fractured rock permeability applied on the basis of these data are shown to be insufficient to explain the higher permeabilities measured in the unconfined Chalk beneath valley axes. In these settings, conduit flow mechanisms must be invoked; this in turn implies that modifications to standard matrix diffusion models are required to describe solute transport accurately.

Using mechanical models to investigate the controls on fracture geometry and distribution in chalk

Geological Society, London, Special Publications, 2014

Chalk is an important reservoir rock. However, owing to its low permeability, fractures are key to producing hydrocarbons from chalk reservoirs. Fractures in chalk usually form one of three geometric patterns: localized fractures (commonly concentric rings) developed around tips, bends and splays in larger faults; regularly spaced regional fracture sets; and fracture corridors comprising narrow zones of closely spaced parallel fractures. Localized fracture patterns are likely to give only local permeability enhancement; regional fracture sets and, especially, fracture corridors may provide long, high-permeability flow pathways through the chalk. Field mapping shows that both localized fracture patterns and fracture corridors often nucleate around larger faults; however, the fracture corridors rapidly propagate away from the faults following the regional stress orientation. It is therefore not necessary to know the detailed fault geometry to predict the geometry of the fracture corri...