"DRILLING FLUID CHALLENGES FOR OIL-WELL DEEP DRILLING Assoc. Prof"
Oil and Gas Exploration DRILLING FLUID CHALLENGES FOR OIL-WELL DEEP DRILLING Assoc. Prof. Vassilios C. Kelessidis1 1 Mineral Resources Engineering Department Technical University of Crete, email@example.com – Greece Paper presented at the INTERNATIONAL MULTIDISCIPLINARY SCIENTIFIC GEO- CONFERENCE & EXPO, Modern Management of Mine Producing, Geology and Environmental Protection SGEM 2009, Albena, Bulgaria, June 14-19. ABSTRACT Continuously increasing demand for energy resources drives petroleum industry to search in very deep waters, drilling very deep wells, encountering adverse conditions of pressures and temperatures and thus having to resolve a multitude of problems. In many cases, the problems can be attributed to the performance of drilling fluids which then presents challenges to drilling fluid industry. Such problems include poor hole cleaning, high pressure losses, loss circulation zones, fluid gelation, reservoir fluid invasions. Thus, drilling fluid industry must work hard to overcome these challenges figuring out solutions which include fluid formulations capable of good continuous performance under all adverse conditions. In this paper we address these issues, we present the adverse conditions in the deep drilling environment of pressures and temperatures, and the implications they have on flow pressures, formation damage and well control. We analyze the approaches taken by different operators in terms of drilling fluid design and implementation in such wells from a series of reported case studies. These include rheological measurements at high temperature and pressures and attempts to model-fit the data to predictive rheological models which could then be used in appropriate hydraulic models to estimate pressure drop profiles along the wellbore. High temperatures present additional stability challenges to drilling fluids which could be overcome with the addition of appropriate additives. The search for such additives is continuous and in this paper we discuss various alternatives, the operational procedures and address also potential problems. Finally, we present possible optimum solutions to overcome such problems and increase the success rate of drilling fluid performance in these difficult conditions. Keywords: drilling fluids, deep drilling, challenges, rheology, mpd INTRODUCTION Oil and gas still remain the inexpensive fuels which turn the world economy. Thus, hydrocarbon exploration has been intense during the past years and industry is now searching in total depths and water depths never thought before. The environment is very hostile and challenging for the drilling industry, characterized with high pressures and high temperatures, hard rock, and presents severe wellbore stability problems [1-4]. Drilling industry has developed drilling technology adequately so that deep wells can be drilled safely, but in order to produce them these wells need to be completed and industry has identified technology development gaps which still require design and 1 International Multidisciplinary Scientific GeoConference SGEM 2009 development . The industry continuously formulates new drilling fluids to deal with such hostile environments and recent reviews have addressed these issues . The drilling industry is changing and avoids previous practices of heavy overbalanced drilling, trying to achieve a balanced drilling scenario, and applying Managed Pressure Drilling (MPD) techniques, monitoring all available parameters, in order to achieve faster drilling rates. For optimum application of managed pressure drilling, availability of drilling fluids capable of maintaining their stability under down hole conditions is essential. Drilling hydraulic simulators are essential and there is a need for fairly accurate simulators and good and appropriate rheological model such as Herschel- Bulkley fluid which describe well the behaviour of these complex fluids . In addition, good hydraulic models are needed to predict pressure drop in annuli for these fluids covering the whole range of laminar, transitional and laminar flow, especially in new approaches like casing drilling , and recent advances make it possible using simple approaches . DEEP DRILLING CHALLENGES Deep drilling presents great challenges to the industry, requires large and expensive rigs, encounters hard rock thus producing very low penetration rates, making the whole effort even more expensive. One of the most recognized drilling challenges for deep drilling and in particular drilling in deep waters is the smaller tolerance between pore pressure and fracture pressure gradient which results in narrow pressure margins (Fig. 1), where it is shown the wider pressure margin for shallow water and the much narrower pressure margin for deep water. Fig. 1 – Pore and Fracture pressure window in shallow and deep water (adapted from Rocha et al.  The close pressure tolerance demanded new approaches to pressure control and the process has been developed, Managed Pressure Drilling, which utilizes friction pressure and annular back-pressure to control difficult drilling environments. MPD procedures do not have open circulation system, rather a close loop circulation system is used (Fig. 2), which, depending on the environment may be designed for shore and offshore applications . These systems allow for the application of back-pressure thereby enabling custom-designed control of pressure along the wellbore. There are several 2 Oil and Gas Exploration MPD applications, starting from underbalanced drilling to constant bottom hole pressure drilling . Some concepts call for dual gradient drilling which is considered as an option, not only for oil and gas production but also for scientific drilling . There are many parameters that play a part in the pressure profile in the wellbore, like density, rheology, flow geometry, injection rates, back-pressure (or choke pressure). The effects of these parameters are different, but interact with one another and full comprehensive models are needed to take them into account . One should also take a closer look at the number of fluids present in the wellbore during MPD drilling because up to four fluids may be encountered simultaneously along the wellbore, gas, liquid, droplets and solids, thus necessitating the correct modelling approach, not only in terms of rheology bur also in terms of hydraulics. Figure 2. Variations of Managed Pressure Drilling for offshore application (adapted from Hannegan, ). MPD allows the operator to dynamically manage any influxes enabling improved well control because it provides flexibility to manipulate annular pressure. Applications of MPD are gaining in momentum and several cases have been noted indicating the benefits of such techniques. For e.g. Perez-Telles et al.  demonstrated the benefits during drilling in Mexico at depths of 6,424 m with a 1.50 SG drilling mud and with MPD, only one trip was performed before reaching total depth, thus saving 30 hours in operation. Bit vibrations also have been identified as deep hard rock drilling problems, while more efficient hydraulics and fluid delivery systems, like, bigger surface pumps and larger- diameter drill pipe, have helped increase drilling efficiency and reduce the costs of drilling during the past couple of decades, and all these require good formulation of drilling fluids and an understanding of the functions of all additives . DRILLING FLUID CHALLENGES Drilling fluids perform a multitude of tasks while drilling, offering hydrostatic pressures, cooling the bit, transporting cuttings to surface, maintaining wellbore 3 International Multidisciplinary Scientific GeoConference SGEM 2009 stability. The drilling fluids must have good stability in terms of density and rheology. In addition, good hydraulics and temperature simulators and phenomenological models are needed to allow predictions of circulating pressures and temperatures, as the operating pressure margins between formation fluid pressures and formation fracture pressures are fairly narrow. However, up until now rheological and density measurements were performed at atmospheric conditions for most of the wells while for more demanding wells, measurements were performed for up to 177 °C and 1400 bar. There are, however, needs, for e.g. in deep gas drilling in the offshore continental shelf of Gulf of Mexico, which may demand measurements at pressures up to 2100 bar and temperatures up to 316 °C , thus new viscometers have been sought to perform such measurements . In Fig. 3, rheograms of drilling fluids measured at different temperatures and pressures, showing the effects of higher as well as lower conditions in the field are shown . Various polymers are added to WBM, OBM and SBM fluids to enhance thermal stability and recent work [16,17,18] has demonstrated the ability of lignite addition to thermal stability of water-bentonite dispersions. Fig. 3. Rheograms of drilling fluids at high temperature and pressures. The effect is significant (from Davison et al. . When applying MPD techniques, a good understanding of the effects of the parameters affecting pressure profile along the wellbore is essential and because the rheology of MPD fluids plays an extremely important role, appropriate rheological models should be used and derived from appropriate viscometer studies, using the readings at all six speeds, and maybe even more, on the viscometer [4,12]. In Fig. 4 an example case for application of MPD with a back pressure is shown. Deep water drilling often uses synthetic-base drilling fluids (SBM) with properties that can address these challenges, because they offer faster penetration rates and enhanced wellbore stability. In cases, though, where loss circulation may result, like the ones encountered in Gulf of Mexico, new innovations are needed and have been developed by the industry like the “Flat Rheology” drilling fluid systems [20,21] which attempt to keep a constant yield point despite changes in temperature and pressure along the 4 Oil and Gas Exploration wellbore, thus effectively cleaning the wellbore with the originally designed yield point. Rheograms for such fluids are shown, for example in Fig. 5, where for the very different temperatures and pressures, yield stress is not varying much, while rheology changes dramatically. Fig. 4. Cases for MPD application: (a) Static pressures are OK, but HH+AFP exceeds formation strength and losses occur. (b) Possible solution, use lower density and impose backpressure when static (adapted from Smith, . (Notes: HH=hydrostatic pressure; AFP=annular flow pressure; BHP=bottom hole pressure; BP=back pressure). Drilling fluid additives which give flexibility when designing and implementing a drilling campaign, are continuously sought. A fairly interesting twist is with respect to density control, which has been achieved till today inexpensively with barite. However, it presents severe technical limitations in deep wells because of barite settling often called barite sag [22,23]. Recent research  has shown that reducing barite particle diameter by 100 reduces settling, or sag, by a factor of 10,000. Thus, the new approach is to grind barite from about 75 μm to about 1-3 μm, and this has delivered field benefits never achieved before by minimizing barite sag but also reducing equivalent circulating density (ECD). At the same time it enhances bit hydraulics and offers superior hole cleaning allowing, thus, to operate in very narrow pressure margins, especially for drilling many mature reservoirs in the North Sea with complex well trajectories and narrow drilling tolerances . In Figure 6, the rheograms for 13.2 ppg (1.59 SG) fluids with regular barite and with micron-sized barite are compared with observation of good rheology. Herschel-Bulkley is the rheological model of choice and the parameters are indicated in the Figure. The analysis shows, though, that the yield stress in the case of micron sized barite is fairly lower than the regular one, and this has to be taken into account. Furthermore, the issue of potential formation damage with such small barite particles must also be addressed. In addition, phenomenological modeling taking into account Herschel-Bulkley rheology should also be sought and not only using Bingham plastic rheological model . HPHT also requires appropriate cement systems and tools which conform to specific parameters, require special design attention, modified testing procedures and new products  in order to withstand the well high temperatures for the life of the well without jeopardizing cement properties. 5 International Multidisciplinary Scientific GeoConference SGEM 2009 Fig. 5. Flat rheology fluid rheograms from two different formulations at different tempearures and pressures (adapted from Mullen et al. ). API barite micron sized barite τ y = 4 . 85 Pa τ y = 0.92 Pa K = 0 . 116 Pa * s K = 0.076 Pa * s n = 0 . 841 n = 0.854 Figure 6. Rheograms for API barite loaded and micron-sized barite loaded, 13.2 ppg drilling fluids (adapted from Oakley . Of course many important issues are not addressed because of the inability to have appropriate models and techniques for dealing with them. For e.g., while fairly expensive instruments have been developed to measure rheology at high pressures and temperatures, what is not really addressed is the calibration of the equipment. All oil field viscometers are normally calibrated with Newtonian fluids, besides the fact that they are used to measure rheological parameters of drilling fluids which are highly non- Newtonian, hence, there is a very strong need for having standards for non-Newtonian calibration, if there can ever be developed. Furthermore, the simple but also more 6 Oil and Gas Exploration complex rheological models used to model non-Newtonian fluid rheological behaviour are normally derived, for oil-drilling industry, using the narrow gap approximation and Newtonian shear rates, while errors arise for all types of models and in particular for Herschel-Bulkley rheological models, as it has recently been demonstrated . Another important property of drilling fluids not taken into account is thixotropy, although, measurements of gel strength at 10s / 10min give a qualitative indication about the thixotropic state of the fluid, while recent advances have addressed this issue for drilling fluids [28,29,30]. CONCLUSIONS The continuous search for more hydrocarbons pushes drilling industry to technical limits and more hardware is needed to enable drilling in high depths where high temperatures and pressures and harder rock are encountered. These challenges also demand better formulation of smart drilling fluids. Drilling techniques involve older and newer procedures, all nowadays referred to as managed pressure drilling techniques which, using different approaches, depart from open systems and present a closed loop drilling fluid circulation system. Such techniques, needed to manage very narrow pressure margins between pore and fracture pressures, rely on good hydraulic simulators capable of predicting with fair accuracy pressure profiles along the wellbore. Good and representative rheological models are also needed for the prediction of the performance of the new type of drilling fluids which can withstand the extreme environment. The model of choice should be the Herschel-Bulkley rheological model which describes well complex fluid rheology and appropriate modeling is necessary for the hydraulics of such fluids. We address these issues and present the challenges ahead. Furthermore, we point out areas that have not been dealt with so far, because in the very more complex and difficult environment, such issues have to be properly addressed and resolved which will enable the industry to develop practices for safer and more economical drilling. REFERENCES 1. R. Bland, G. Mullen, Y. Gonzalez, F. Harvey, M. 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