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Chalk, due to its low permeability, requires stimulation of the near-wellbore area to create conductive flow paths for the fluid flow. Matrix acidizing and acid fracturing methods are well-known solutions implemented in the North Sea chalk reservoirs; however, the methods not always give the desired stimulation effect. High cost of the frac operations affects the project economics and well payback. Thus, there is a demand for new cost-efficient technologies. In this thesis, Radial Jet Drilling (RJD) stimulation technique is investigated in order to contribute to the economics of energy recovery from chalk reservoirs in a more controlled and affordable manner than those conventional methods used in the industry as fracturing and acidizing. The RJD technique is based on drilling multiple lateral boreholes from the main bore, in which a power of focused jet from a small orifice of the nozzle is used to break down the rock enabling a deep penetration into the formation. The chalk formations in many North Sea oil fields are high porosity materials, composed of particles, which are mainly single crystal laths of calcite produced by the disaggregation of coccoliths of size in the range of 0.5-3 microns. Based on laboratory testing results, chalk has a complex mechanical behavior, yielding a non-linear stressstrain response not only due to stress-induced deformation, but also due to timedependent deformation at constant stress loading. Rock mechanics experiment results reveal several yield mechanisms occurring in chalk, such as shear failure, pore collapse and tensile failure. Thus, designing an open hole completions is difficult, where a combination of geomechanics behavior of the material in addition to fluid flow conditions need to be taken into account. Effective stress changes around boreholes due to drilling and hydrocarbon extraction operations raises a concern, especially when high drawdown and depletion taking place in the reservoir resulting in fines production. Practical application of the RJD technique in petroleum and geothermal reservoirs has long been limited due to uncertainties related to the influence of rock mass strength (stiffness), geological heterogeneity, in situ conditions in the field, fluid effect, and nozzle type, among others, on the jet drilling performance (penetration, hole geometry, and lateral stability). Chapter 2 of this thesis examines the jet drilling operational parameters such as nozzle type, fluid type and the reservoir host rock characteristics, such as inherent rock mechanics properties, porosity, permeability, and in-situ stresses on the jet drilling performance, such as drilling rate and the hole geometry. Two types of outcrop chalks, a soft Austin (US) and tight Welton (UK), are studied as analogues to the reservoir chalk in the North Sea. With this study, we have identified that there exists a correlation between the rock strength (and stiffness) and the jet drilling performance. For the tight chalk a higher minimum injection pressure was required compared to that of soft chalk. Soft chalk presents larger borehole size and better rate of penetration, both with water and acid aided fluid, due to its higher matrix permeability value, as well as lower strength (stiffness) properties that favour more penetration of the jet drilling fluid into the rock and faster erosion/breakage compared to tight chalk. Static nozzles create a larger contact area compared to rotating nozzles. Jet drilling in confined stress condition appears to be faster due to localization of shear failure around the drilled hole induced by the differential stresses compared to the jet drilling at unconfined stress condition. In Chapter 3, impact of the high velocity jet on the alteration of the rock mechanics properties of chalk is studied. Intact and jet drilled specimens were analyzed under a series of standard rock mechanics tests as well as investigated on the microstructure change under Scanning Electron Microscopy (SEM) analysis. The tested chalk specimens that were jet drilled under confined condition with acid aided fluid showed the most significant alteration of the rock mechanics properties. At the radius of up to about 4 cm surrounding the jet drilled hole, a significant weakening of strength (tensile, uniaxial/triaxial compressive and pore collapse stress) and stiffness properties was observed. These specimens under SEM analysis showed post jet drilling effect as numerous micro-perforations created on the surface of microsparites (calcite). The observed weakening effect of the confined jet drilling on the strength and stiffness properties of the chalk is likely due to local stress concentration from confining pressure near the jet drilled area and partly due to the acid dissolution effect. Evaluation of the feasibility and stability of openhole RJD laterals under in-situ operational conditions requires a detailed geomechanics analysis. Moreover, a modelling tool for predicting the long term stability has to capture the chalk’s complex mechanical behavior as well as its yield mechanisms. In this context, a methodology for modelling wellbore stability of chalk was developed (Chapter 4) that was based on five main parts: 1. The rock mechanics testing for estimating the rock properties; 2. an advanced rock mechanics testing method, called the single lateral hole (SLH), to study the wellbore stability under various loading, creep and flow conditions; 3. utilizing CT imaging for identifying the damaged zone and its extension, and 4. backward numerical simulations of the test data to improve the estimated rock mechanics properties and 5. forward numerical simulations utilizing the estimated properties to predict the stability of open-hole in chalk under reservoir in-situ and operational conditions. In chapter 5, the methodology presented in previous chapter was used to evaluate the hole stability of the radial jet drilled laterals in the reservoir chalk. Oil-bearing Ekofisk chalk formation in the Gorm field, located in the Danish part of the North Sea, was the focus area of feasibility study of the RJD stimulation technique. The main objectives were 1. to assess the sustainability of the laterals under simulated pressure drawdown and reservoir depletion schemes in the long-run; and 2. to evaluate influence of the transient fluid pressure gradient, generated during shut-down and bean-up operations on the stability of the laterals. It was concluded that during the first year of production under simulated drawdown and depletion schemes, the lateral hole remained stable with a small development of the plastic strain at theborehole wall. Continuing simulation at constant stress phase (creep) resulted in the development of shear failure breakouts, that expanded further into the chalk with time. In some distance from the borehole wall, stress concentration was observed and porosity reduced in this region, suggesting that pore collapse occurring. At the wall, the porosity increased, corresponding to the volume increase due to dilatancy. Closer to a four years of simulation under constant stress, the hole was likely to change its geometry by removal of the plastified area. The effect of the hole geometry with wings was also studied. Results of the simulation showed that wings close shortly (in several hours) after the jet drilling; however, the main lateral hole should behave as described above. With flow test two different injection rates were studied and test data showed insignificant permeability change under the generated transient pressure gradient. In conclusion, the viability of the RJD technique in the chalk reservoir seems reasonable. Experiments with jet drilling on both tight and soft chalks have shown the feasibility of this technique to create lateral boreholes. In soft chalks, jet drilling with acid aided fluid may pose concerns in regards to the weakening of the near lateral rock, thereby leading to an earlier closure of the hole than that drilled with water. In terms of stability of the hole in the long-run, based on the simulation analyses for the specific field, the lateral can serve without instability concerns up to a year. However, with time, depending on the operational conditions of the reservoir, such as drawdown rate, depletion, how aggressive the shut-in and shut-down processes, among others, will affect the performance of the hole.
|Publisher||Technical University of Denmark|
|Number of pages||184|
|Publication status||Published - 2020|
01/10/2016 → 11/03/2020