Per Horsrud

Leak-off tests for horizontal stress determination?

Determination of in-situ principal stresses still represents a major challenge to the oil industry. Among the methods proposed to estimate horizontal stress magnitudes and directions is the inversion of results from leak-off tests. Previously published work with this method has been based on a procedure where all shear stress components were neglected. This paper presents a procedure based on the complete stress solution and user input from three data sets. Results based on the complete stress solution may in many cases differ considerably from the simplified (linearised) version. The results presented in this paper show that even if a complete stress model is used, the results may contain large uncertainties. These uncertainties are related to several factors, such as the uncertainty of leak-off pressure measurements, stress field homogeneity and the boundary conditions at the borehole (permeable/impermeable borehole wall, existence of cracks and fractures, etc.) during leak-off. Evaluation of the horizontal stress field should therefore be based on several methods and several sources of information.

Local rock mechanical knowledge improves drilling performance in fractured formations at the Heidrun field

Abstract Despite the use of inhibitive water-based mud (KCl), high pump rates to obtain good hole cleaning and relatively high mudweight, problems with cavings, pack-offs and lost circulation persisted when drilling Tertiary shale formations in the Heidrun field offshore Mid-Norway. A study was therefore initiated, including extensive data collection (logging and coring of the shale) and subsequent core testing, data analysis and evaluation of field experience. The study showed that the major problem zone was not the low-density zone as anticipated. However, the core revealed a fractured and crushed zone, which was eventually found to be the main source of the problem. The study has resulted in a new strategy. An improved borehole stability model has been established, and the mud and operational strategies have been revised. The mudweight has been reduced, the salt content of the mud has been reduced and good hole cleaning is obtained through a low-viscosity mud, which is kept close to turbulent flow. Furthermore, if possible, wells are now planned without steering in these formations. If steering cannot be omitted, 3-D rotary steerable systems are utilized. An economic evaluation of the study was also performed, showing a substantial benefit/cost ratio. So far, the cost reduction for an average well is close to 20 MNOK (ca. 2.5 million USD). With more than 50 wells left to drill in the field, the potential for cost-saving is large. Additionally, there is a huge and non-quantified effect from accelerated production.

Particulate-Based Loss-Prevention Material--The Secrets of Fracture Sealing Revealed!

Owing to the narrow drilling margin that exists between th e pore pressure and fracture pressure gradients, drilling in depleted reservoir, HPHT and deep water environments is univer sally recognized as being technically challenging. A number of field techniques are available for mitigating against many of the drilling problems encountered. Included amongst these are specialized fluid engineering that involve use of chemical- and particulate-based treatments for minimi zing or preventing losses. In many instances these techniques can be used to strengthen or stabilize the wellbore when drillin g on or near the fracture gradient thereby potentially eliminating the need for intermediate casing strings. This paper discusses particulate-based treatment design for sealing fractures. Substantial experience gained from innovative laboratory testing has highlighted the mechanisms and many factors that determine the effectiveness of the fracture seal. The particle size distribution relative to the fr acture aperture, particle morphology, volumetric concentration, fluid rheology and fluid-loss-control influence whether the seal is established within the fracture or at the fracture mo uth. Understanding this distinction is important with respect to selecting the optimum treatment and its application for giv en field conditions. Parameters critical for optimizing the treatment h ave been identified and are discussed in the context of laboratory and field experience.

Shale Porosities As Determined By NMR

We present NMR measurements on three different shales from the North Sea. The motivation was to investigate whether one may employ NMR, which is a fast, non-destructive analytical tool, to measure the effective porosity of lowpermeable shales. The latter porosity is relevant for possible correlations to permeability. Presently, log-derived porosities (neutron, density) tend to yield closer to the total porosity which may differ significantly from in shales. Core porosities may be determined by means of timeconsuming, destructive drying-up techniques, restricting the access to the cores. Our measured NMR-porosities seem to agree with those attained by the loss of water. However, there may occasionally arise an extra NMRcontribution from the “dry” sample. This part may tentatively stem from bound or confined water within the shale. Even though more work remains to be done, NMR has proven its potential as a tool for petrophysical measurements on shale.

Mechanical Anisotropies of Shale

Laboratory experiments have been performed with a number of shale samples, including commonly studied outcrops like Pierre and Mancos Shale. The angular dependences of unconfined strength, friction angle, static E-modulus and P-wave velocity have been measured and modelled. The anisotropy of the ratio between dynamic and static moduli is found to represent plastic / brittleness anisotropy. Stress and stress path dependent velocity anisotropy will be shown and related to possible 4D seismic applications. Implications for field applications to borehole stability and fracturing will be discussed.