The challenges found in deepwater and ultra-deepwater drilling have, in a remarkable short period, forced the oil industry to develop new significant technologies and techniques. The characteristics of the deepwater environments have pushed design criteria, normally used in onshore and shallow water wells, to values beyond their traditional limits. All drilling phases of deepwater and ultra deepwater wells face challenges. The initial phases, generally composed of soft soil or just mud, have required a lot of experience in terms of jetting the conductor pipe to avoid sinking of the wellhead. In the intermediate phases, engineers must be very careful to avoid lost circulation due to the narrow window between pore pressure and low fracture pressure gradients. Besides, well bore instability, always an issue for directional drilling, often limits the length of the deepwater well departures to values considered small if compared to those obtained in shallow waters or onshore. In addition, the drilling of permeable rocks, many times just loose and unconsolidated sands, increases the chance of differential sticking. To complete the picture, watching closely well operation and drilling parameters to keep risks under control generally is not enough. Creativity is very often necessary to overcome the ultra-deepwater challenges. This paper describes Petrobras drilling experience in deepwater and ultra-deepwater. A number of historical cases are shown to illustrate the main obstacles an Operator normally faces when drilling in deepwater. The evolution of well design and drilling practices as the result of the increase of the water depth and its related problems, are also presented. Several experiences with new tools, rig performances and drilling fluid products specific for deepwater are described. Finally, deepwater projects that include extended reach drilling and other special studies are also discussed. Introduction The discovery of important deepwater oil fields such as Marlim and Albacora has inspired Petrobras to look for new frontiers located in deeper waters. However, initially thought as simple adaptation of the work done in shallow water, a number of new challenges also followed this move. According to the oil industry, deepwater wells are those wells drilled in water depths ranging from 300 meters to 1,500 meters. Wells located in water depths higher than 1,500 meters are classified as ultra deepwater wells. Deepwater drilling with dynamic positioned (DP) rigs has been a reality in Brazil since 1984. Up to 1991, Petrobras used only these types of rigs to drill exploratory wells in the Brazilian Basins, including Campos Basin. After having drilled a number of wells in water depths higher than 1000 meters, Petrobras has started in 1990 an extensive program to drill development wells in deepwater oilfields using both, DP and anchored rigs. Fig. 1 shows a schematic map of the Brazilian Coast and the distribution of deep and ultra deepwater wells along it. Campus Basin, where the main oilfields are located, has the majority of the drilled wells. Though, Espirito Santos Basin, where a number of new discovery have been announced by Petrobras, is expected to have a considerable increase of drilling activities in the future years. Fig. 2 displays the number of wells drilled in deep and ultra deepwater versus time in years.
Summary Projecting safer and more economic wells calls for estimating correctly the fracture pressure gradient. On the other hand, a poor prediction of the fracture pressure gradient may lead to serious accidents such as lost circulation followed by a kick. Although these kind of accidents can occur in any phase of the well, drilling shallow formations can offer additional dangerous due to shallow gas kicks, because they have the potential of becoming a shallow gas blowout leading sometimes to the formation of craters. Often, one of the main problem when estimating the fracture pressure gradient is the lack of data. In fact, drilling engineers generally face situations where only leak off test data (frequently having questionable results) are available. This problem is normally the case when drilling shallow formations where very few information are collected. This paper presents a new method to estimate fracture pressure gradient. The proposed method has the advantage of (a) using only the knowledge of leak off test data and (b) being independent of the pore pressure. The method is based on a new concept called "pseudo-over- burden pressure", defined as the overburden pressure a formation would exhibit if it were plastic. The method was applied in several areas of the world such as U.S. Gulf Coast (Mississippi Canyon and Green Canyon) with very good results. Introduction Overburden pressure gradient is defined as the pressure variation by depth due to the weight of the rocks' matrix and fluids in the rock pore spaces. If the bulk density (Pb) is a known function of depth, the overburden pressure for each depth can easily be calculated by integrating bulk density function versus depth. Pore pressure gradient is the pressure gradient of the fluid contained in rock pore space. The fracture pressure gradient is defined as the pressure gradient that will cause fracture of the formation. In other words, if a formation is exposed to a pressure higher than its fracture pressure limit, the formation will fracture and a loss of circulation will occur. Extreme problems related to loss of circulation can vary from well collapse (due to the decrease in hydrostatic pressure), to a quite severe gas kick (also due to the decrease in hydrostatic pressure) followed by a underground blowout. The consequences of an underground blowout are unpredictable. In the best scenario, the formation fluid will stay confined underground; however, it may migrate toward shallow and unconsolidated sediments resulting in a crater. Collectively, these aspects make formation fracture pressure knowledge fundamental when drilling oil wells.
Currently some of the most challenging wells being drilled by our industry are located in deepwater zones in the GOM. Many of those wells are in water depths of 8,000 ft. or more and several are targeting reservoirs around 30,000 ft. and beyond.Drilling engineers face many challenges when planning drilling and completion operations for such wells. There are not many rigs available to drill in ultra-deepwater, and even the modern rigs capable of operating in this environment will present limitations ranging from the maximum mud weight possible to be used, due to riser restrictions, to the hook load capability to run very heavy intermediate casings that will easily surpass one million pounds.The well itself will present many problems including high pressure and high temperature formations, the need of multiple casing strings, unstable formations, hole cleaning, unexpected presence of tar zones, huge layers of salt, the need to underream the well, difficult to do an efficient evaluation program etc.On the completion side, the challenges can be even more demanding, with the need to complete multiple zones while trying to minimize future expensive workover operations. This paper presents some practical experiences on dealing with various of the abovementioned problems and also suggestions to make the problems manageable. It might be emphasized that we do not have a perfect solution to all problems and that we are far to have the most efficient solution to drill deep wells located in ultra-deepwater zones in the GOM. However, with daily operational costs reaching one million dollars or more, it is our intention in this paper to discuss the problems, to point out some possible directions, to show some field cases and to open a discussion that might benefit the entire industry.
This paper was selected f a presentation by h e OTC Pmgram Canmittee following review of M a a t i o n contained in an abstrecl submitted by the autws). Contents of t k a peper, as pmwnbd, have rot been reviewed by the Offshore Technolo~y Confersnce and are W e c t to carsdim by the autha(s). Tha material, as presented, does rot w l y rdlect m y poWn of Vw, Offstmre Technology Conference a it8 offken. E m mpmdwkm, w o n , or storage of any pad of this paper f a commercial prrporee wi(hout the written camant of the Offshore Technology Conference is prohibited. Pflission to mpmdse in pht is nrtricted to m abstracl of n d more than 300 wads: i l l -may rpt be cqbd. ubtrac! must contain conspicuous acknowledgmeri4 of where and by whan the peper W a pmemted.New well design and drilling follow-up methodology, based on a modified kick tolerance margin, was developed and successfully applied to the execution of six deep water and five high temperature, high pressure (HTHF' ) exploratory wells. This approach considers the whole scenario in a more realistic way, assuring safe and cost-effective operations. This work makes a review of current methods, introduces the new one and presents three case histories, two in deep water and one in a HTHP environment.In the first case, well PAS-25 at a water depth of 1214m, the application of the new methodology made possible to eliminate an entire casing string, bringing about considerable cost reduction. In the second case, well CES-Ill, it was observed that fiiction pressure loss through the choke line (FPLCL) was very restrictive, requiring special attention during kick tolerance calculations. Finally, the proposed casing setting depth design and follow-up methodology was applied to plan BSS-78, a HTHP exploratory well of Santos Basin, southeast coast area of Brazil. This plan was based on data from a problematic reference well, BSS-70. The obtained results show significant economic advantages.As a result, the experience fiom these field applications indicates that: (1) the determination of reliable pore pressure and fiacture gradient curves is critical for planning and execution purposes; (2) specific drilling and well control procedures have to be defined, based on how critical is the well under consideration; and (3) rigorous training program is necessary for the whole team.
fax 01-972-952-9435. AbstractThe majority of the petroleum engineers agree that fracture pressure gradient is one of the most important items to be considered when designing or drilling a well.The perception of the importance of the fracture pressure gradient comes from the results of the severe economic losses that the oil industry has faced when dealing with lost circulation related problems. In the worst scenario, these problems can escalate to a blowout due to the reduction of the hydrostatic pressure in the well. In addition, lost circulation problems are likely to occur and become even harder to control if the well is in deepwater.It is well known that a correct prediction of the fracture pressure gradient minimize drilling problems. However, the methods used by oil industry to perform this task are generally based on equations or methodologies that give questionable results and do not match actual field data.Fracture pressure gradient methods are generally based either on equation derived from rock mechanics theories or in simplified methods. Although, the first tries to closely represent the rocks underground behavior, they are too complex and call for a number of data that normally are not available. On the other side, the second carries many simplifications and barely represents subsurface conditions. However, the last is simple to use and consequently more popular among drilling personnel. Regardless of the method, a good calibration, in general hard to be accomplished, is always necessary to provide good estimates.Finally, performing leak off tests (LOT) is usually the procedure carried out by most of the oil companies to establish fracture pressure gradient values for a given area. Once LOT`s are obtained, they are used to calibrate simple equations or will be part of the company database to simply build fracture pressure gradient curves.The objective of this work is to make a critical examination of the current methods used to estimate fracture pressure gradient. The work also presents simple methodologies based on leak of tests data to estimate fracture pressure gradient for a given area. Results based on actual field data to exemplify the use of the presented methodology will be shown, mostly for deepwater oilfield.
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