TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractA series of laboratory experiments were conducted to investigate the influence of overburden and in-situ stresses on non-Darcy gas flow behavior in Dakota sandstone. Nitrogen was flooded through cylindrical core in a triaxial core holder under specific condition of temperature at 100ºF, with core outlet pore pressure at 500 psi, axial and radial stress from 2,000 to 10,000 psi, and. nitrogen reservoir pump pressure at 2,000 psi with pump flow rates from 25 to 10,000 cc/hr at 80ºF. Permeability and non-Darcy coefficient were determined using Forchheimer's method. It was found that with the increase of overburden and in-situ stresses, permeability decreases while non-Darcy flow coefficient increases. Average effective normal stress and shear stress were used to quantitatively express the influence of overburden and in-situ stresses. It was found that average effective normal stress has a good linear relationship with both permeability and non-Darcy flow coefficient. In contrast, average shear stress did not appear to influence the permeability and non-Darcy coefficient.
The purpose of the paper is to present the variation of total ozone concentration over Dumdum at Kolkata, India during different seasons. It is concluded that the trend of ozone concentration during different seasons are different. A critical analysis is done and following important results are obtained. (i)The total ozone concentration increases during the pre-monsoon and winter periods. During premonsoon period rate of formation of ozone increases sharply with the rise of surface temperature.(ii) TCO decreases during the monsoon and post-monsoon periods, throughout the period of study. During monsoon period rate of depletion of ozone increases and during post-monsoon period rate of depletion of ozone decreases with the increase of temperature.(iii) Possible explanations are also presented.
This paper describes application of openhole sidetrack (OHSDTR) technique to drill deep gas multilateral horizontal wells in Saudi Arabia. The drawbacks of earlier whipstock exits were studied and an alternate technique was proposed and implemented. The typical approach to drilling Khuff dual laterals was to drill the lower lateral and then set whipstock, and cut a window in the base Khuff-B and drill the upper lateral. Various problems observed using this technique were:Problem setting whipstock.Premature setting while running in holeProblem cutting the window. Mill twist offs leading to a long fishing operation; in some cases setting another whipstock and cutting another window.Milling the top of the whipstock while cutting the window leading to a severe problem in retrieving the whipstock. Whipstock from different companies were tried, however, we could not get rid of multiple failures while milling and retrieving the whipstock. Even with modifications, a new whipstock system was tried in which milling was done in two parts; a first run with the milling machine to mill only the casing, and a second run with different mills to cut the window through to formation, but problems continued. Saudi Aramco and Schlumberger D&M came up with the idea of drilling dual laterals using an OHSDTR technique to avoid complication due to whipstock failure. Well HRDH-XYZ was the first successful dual lateral horizontal Khuff-C gas producer drilled with the open hole sidetrack technique, in a record time of 98.8 days against the 165 days planned, the technique saved $975,000 and two weeks of rig time. This was a step change in drilling practice, to drill dual laterals saving considerable rig days to Saudi Aramco, and removing the time consuming whipstock operation to the extent of losing the well, due to unsuccessful whipstock retrieval. This technique brought a transformation in the way dual lateral wells were drilled in deep gas drilling in Saudi Arabia, later on similar technique was applied in drilling dual laterals in Khuff-B reservoir as well. Since the introduction of this technique on well HRDH-XYZ, 50 wells have been drilled successfully saving (approximately) 735 days to Saudi Aramco. This paper further details challenges; lesson learnt and success story about introduction of Open Hole Sidetrack in deep Gas drilling.
Drilling horizontal gas wells at an average TVD of 12,500 ft in Saudi Arabia has always been very challenging owing to harsh drilling conditions. These wells are drilled in southern Ghawar field. Subject formations include bands of Lower and Middle Triassic and Upper Permian, composed of dolomite, shale, silt stone, and anhydrite formations. The main challenges in drilling these formations are slower penetration rates and difficulty in sliding with conventional motors in both the build and lateral sections. This paper describes a step change in drilling performance using powered rotary steerable systems (PRSS) technology that led to record directional runs through difficult Khuff sections. With the PRSS assembly, the company was able to drill both vertical and curve sections in one run, maximize ROP, and increase footage per bit run while meeting directional requirements. PRSS successfully kicked off from vertical and improved ROP up to 192% compared to conventional motor ROP in deep gas drilling. Significant improvements in ROP resulted in saving multiple bit trips in 12-in., 8 3/8-in., and 5 7/8-in. hole sections. Introduction: Figure 1 shows the stratigraphic view of Ghawar field in Saudi Arabia. The late Permian Khuff A, B, and C stacked carbonate reservoirs are the main gas-producing zones at depths of 10,000 to 12,000 ft. Formations encountered are well recognized for providing a harsh and hard drilling environment. Steerable motor drilling is a relatively inefficient process, with associated problems in the area ranging from trajectory control in unstable formations to slow penetration rates, pipe sticking, and slide drilling. Conventional steerable motor drilling requires sliding of the bottomhole assembly to steer the well path; therefore, drilling becomes slower and potentially more problematic. Rate of penetration (ROP) is impacted as a result of wellbore friction, plus BHA and drillstring components tend to hang up. Also, while drilling the Khuff wells, high mud weights are required to balance the formation pressures. Drilling formations with different reservoir pressure increases the tendency of the drillstring to stick to the permeable formations, hence giving rise to differential sticking, which in turn gives rise to sliding difficulties. Conventional Well Profile and Casing Design Two different types of casing designs are used while drilling these deep gas wells, namely "K1" and "K2". Figure 2 shows the comparison of the casing designs.
A series of laboratory experiments were conducted to investigate the influence of overburden and in-situ stresses on non-Darcy gas flow behavior in Dakota sandstone. Nitrogen was flooded through cylindrical core in a triaxial core holder under specific condition of temperature at 100ºF, with core outlet pore pressure at 500 psi, axial and radial stress from 2,000 to 10,000 psi, and. nitrogen reservoir pump pressure at 2,000 psi with pump flow rates from 25 to 10,000 cc/hr at 80ºF. Permeability and non-Darcy coefficient were determined using Forchheimer's method. It was found that with the increase of overburden and in-situ stresses, permeability decreases while non-Darcy flow coefficient increases. Average effective normal stress and shear stress were used to quantitatively express the influence of overburden and in-situ stresses. It was found that average effective normal stress has a good linear relationship with both permeability and non-Darcy flow coefficient. In contrast, average shear stress did not appear to influence the permeability and non-Darcy coefficient.
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