No abstract

Thm paper was selected for presentation by an SPE Program Commmee following rewew of mformal!cm ccmamed m an abstract subnwled by the author(s) Con!ents of the paper, as presented, have nol been reviewed by the SO.aety of Petroleum Engineers and are $u@ect to correctmn by the author(s I The mafenal, as rrasented, does not necessarily reflect any pas!bon of the ?,ocaetyof Petroleum Engineers, Its offcers, or members Papars presented at SPE meetmgs am Sub)ect to pubhcat,on rev,ew by Edttorlal Committees of the S.xiety of Petroleum Engneem Permmmon to WY IS restrmted to an abstraci of not more than 3CY3words Illuslrabons may no! be copied The abstract should wtar conspu?dous acknOP.WdgMent of where and by whom !he paper was~esenled VWIte Llbrarlen, SPE P O Box 833636, Rchardson, TX 7W33-38M, U S A fax 01-214-952-9435 Abstract Xanthan Gum is the most popular biopolymer used today to provide hole cleaning and suspension capabilities to water based drilling fluids but it presents some limitations as regards temperature sensitivity and tolerance to field contaminants. In this paper a Scleroglucan is proposed as a better alternative to Xanthan Gum for drilling fluid compositions. Superior benefits offered by Sclerogiucan on hole cleaning, rate of penetration, temperature sensitivity and mud tolerance to shale have been demonstrated by successful field applications and by extensive laboratory studies. Low shear rate viscosities at 0.06 see-l and IJerschel & Bulkley yield stresses are used to quantify hole cleaning capabilities and critical MBT of lab formulations and field muds. Evaluation of cleaning efficiency and cutting concentration is performed with an in house developed program using the Walker and Mayes theory modified with the Herschel &Bulkley theological approach") The oscillation parameters Storage Modulus (G'). Loss Modulus (G") and Loss Tangent (G''/)') are used to characterize the "undisturbed" structure and thixotropy of the systems. According to existing theoriesq['-~) and to the results of' the authors field experience and laboratory studies with analogous form ulations~2, Loss Tangent and Storage Modulus are related to the degree of structuration and to the "strength" of the structure that develops in biopolymer solutions and muds under static conditions. As a consequence, these data are analyzed to determine the suspension capability and resistance to shale contamination of the systems, On the basis of field experience and laboratory studies the main advantages offered by Scleroglucan for drilling muds compositions are confirmed and underlined. The reported field data refer to the first application of Scleroglucan in a Potassium Formate based mud. In the same field other wells were drilled using Scleroglucan in different operative conditions and mud formulations and always with good results.The discussion of these data will be reported elsewhere~'. 105

No abstract

The current need of drilling ultradeep wells involves facing new technical problems, especially concerning the effects of high pressure and temperature values. When acting along all the vertical profile of the well, the pressure and temperature effects heavily influence the rheology of the drilling fluids. Some of the main drilling parameters which are involved are cutting lifting and hole cleaning efficiencies (resulting both from the variation of the velocity profile of the fluid flow, and from the variation of the rheological parameters), and - of course - the pressure spatial distribution along the well profile. In order to characterise the real influence of pressure and temperature upon the rheology of the drilling fluids, circulation tests were performed and repeated at different depths inside cased hole (9 5/8" csg at 2973 m) with the 8 1/2" bit off bottom, while making trip (e. g. for the drilling out task of the casing shoe). Complete sets of data are available of circulation tests performed at 2400 m, 1400 m, 800 m and 198 m of the bit depth (while making the POOH trip), obtained by recording the mud flow rate and the corresponding stand pipe pressure (SPP) values. By developing an original numerical procedure able to determine the equivalent rheological tern (n, k and o of any drilling fluid flowing in the well) which fits the observed SPP data in the best way, it has been possible to characterise the rheological curves corresponding to the different testing depths. The processed rheological outputs are finally analysed and matched, pointing out the effects of pressure P and temperature T. The outcoming effects can be remarked with respect to the equivalent viscosity of the considered drilling fluids and to some of the main practical drilling parameters, such as the velocity profile and the pressure spatial distribution along the well profile. Qualitative trends of the field rheological data versus depth have been compared with the results of laboratory studies performed with an Huxley & Bertram HPHT rheometer with OBM systems. Some interesting analogies have been found with the two different approaches and the changes of the rheological parameters vs depth have been discussed also with reference to the mud structure sensitivity to P and T. Introduction The rheological properties of drilling muds under downhole conditions may be very different from those measured at ambient pressures and temperatures at the surface. High temperatures and pressures can influence the rheological properties of the drilling fluids in several way:–Physically: decreases in temperature and increases in pressure both affect the mobility of the systems and lead to an increase of apparent viscosities and viscoelastic relaxation times. The effect of pressure is expected to be greater with oil based systems owing to the oil phase compressibility.–Electrochemically: an increase in temperature augments the ionic activity of any electrolyte, and the solubility of any partially soluble salt that may be present in the mud. This could alter the balance between the interparticle attractive and repulsive forces and so the degree of dispersion and flocculation of the mud systems. Sometimes, this can also deeply affect the emulsion stability of oil based muds. All these phenomena have a profound impact on rheological properties, especially as far as viscoelasticity and thixotropy are concerned.–Chemically: all hydroxides react with clay minerals at temperatures above 90 C. With many kinds of muds, this can result in a change of the structure and therefore also in a change of the mud rheological properties, Because of the large number of variables involved, the behaviour of drilling muds at high temperatures and pressures may be very complicated so that it can be very difficult to get general guidelines for each group of muds (water base muds, oil base muds, etc.) or even for the same type of mud (little differences in the composition can result in considerable differences in the rheological behaviour). P. 115

Summary This paper discusses the field results of innovative K-Acetate or K-Formate mud formulations that have been used by ENI S.p.A./Agip Division to drill several wells through very plastic shales in South Italy. Field muds have been carefully designed and evaluated as far as drilling and waste disposal activities are concerned. While drilling, the integration between field observations, standard laboratory tests, and nonconventional rheological approaches provided the assessment of useful correlations between rheological properties, performance, and formulations of field muds. These findings permitted a gradual improvement of the mud effectiveness with a large reduction in dilution rates, bit balling, and hole cleaning problems. At the rig, the strict cooperation between all the people involved in field operations represented a key factor for the successful application of the know-how acquired. With respect to the previous wells drilled in the field with traditional dispersed muds, the optimization and careful management of the new muds contributed, in spite of the increased ROP, to a considerable reduction both in time spent to remove bit balling or reaming and in tons of wastes produced per hole volume. These improvements led to great savings in drilling and disposal costs that largely compensated the 8.4% increase in the mud mixing costs per hole volume due to the presence of potassium salts. Introduction Potassium acetate KC2H 3O2 or simply KAc) has been proposed and successfully applied since 1986 as a more environmentally acceptable alternative to potassium chloride (KCl) for drilling fluids.1,2 Potassium chloride, KCl, provides levels of potassium (52% by weight) similar to those provided by KAc (40% by weight) but the high chloride concentrations, associated with KCl, limit the polymer selection and have a harsh impact on plant life. In many areas, these environmental concerns recently imposed drastic restrictions on the chloride concentration accepted by the Italian local regulations in the drilling waste volumes. Recently, another potassium salt, potassium formate (KCOOH), has been proposed and applied in brines and drill-in fluids formulations. 3–10 The formate salts are of increasing interest because they are biodegradable and have a low toxicity to aquatic organisms. In addition to that, they display very little corrosiveness towards ferrous-based metals used in oilfield tubulars and ancillary hardware,8 they have an unusually high solubility in water and reduce the rate of hydrolytic and oxidative degradation of many viscosifiers and fluid loss agents at high temperatures.9 Drilling polymer muds for nonproductive zones, that include in their formulations low concentration of potassium formate as an alternative to the usual KCl, have been first extensively applied only to ENI S.p.A./Agip Division wells. In ENI S.p.A./Agip Division, K-Acetate and K-Formate polymer muds have been used to drill 11 wells located in two different fields (A and B) in south Italy. The muds have been sampled every hundred meters of penetration and analyzed with standard procedures and nonconventional rheological tests, such as low shear rate and oscillatory measurements. These tests helped correlate field muds suspension capability, carrying capacity and resistance to shale contamination to the initial mud formulation and, in particular, to the type and concentration of salts and polymers. The impact of mud formulation and management on the results of discharge operations has been evaluated with reference to some technical and economical indexes that have been defined by means of a statistical study performed on more than fifty wells. The improvements obtained in field B, with the introduction of the Scleroglucan biopolymer in the muds formulation and with the application of the zirconium citrate (ZrC) as a rheology modifier, have not been discussed in this paper because they represented the main subject of previous works.11,12 Experimental Nonconventional Rheology. All the field muds have been examined with the Fann 35 at the minimum and maximum well temperatures. Fann readings have been elaborated with numerical methods to calculate the Herschel Bulkley (H & B) rheological parameters. A Bohlin VOR has been used to perform low shear rate and oscillatory measurements with a couette geometry having the gap 100 times greater than the maximum dimensions of the solids particles contained in the samples. Measurements as a function of shear rate ("rate sweeps") have been carried out by varying the shear rates from 0.02 to 600 s?1 . Oscillatory measurements have been repeated every 30 s for a total time of about 10 min, at a fixed frequency (1 Hz), after the initial imposition of a shear rate of 100 s?1 for a period of 60 s ("time sweeps after a shear history"). Mud Formulations. The mud type and density used in the field A are illustrated in Table 1. The table also reports data relative to three wells previously drilled in the same area with traditional dispersed muds. As regards the new K-polymer systems, the typical mud formulations were prepared at pH 9/9.5, with Xanthan Gum biopolymer as primary viscosifier and suspending agent, polyanionic cellulosic (PAC) and Starch for filtration control, potassium salts (7-14 lb/bbl) and barite to the required weight. When necessary, rheology and filtration properties have been adjusted by controlling the mud degree of flocculation with dilutions or chemical corrections. Filtration control of the high salinity muds (with potassium salts over 7 lb/bbl) sometimes required an increase in biopolymer concentration.