Well Design

Over the next decade a significant amount of exploration and new field developments will take place in salt provinces around the world - in the deepwater Gulf of Mexico, and offshore Angola, Brazil, and North and West Africa. Salt formations provide both opportunities and challenges to the design and construction of the often complex wells to be drilled in these locations. An overview of the many geomechanical considerations necessary to ensure successful well construction when drilling in through-, sub- and near-salt environments is presented. The structural styles of deformed sediments adjacent to salt, combined with stress perturbations caused by the presence of salt, are used to assess the risk of encountering zones that might cause wellbore instability or lost-circulation problems. Well design examples are provided that show how near- and through-salt uncertainties may be included within a geomechanical well design for required mud weights while drilling. Salt is found in many hydrocarbon basins around the world. Significant deposits exist in the Gulf of Mexico (GoM), offshore West Africa and Brazil, in the Southern North Sea, Egypt, and the Middle East (Figure 1[1]). In deep water offshore North America, the GoM and offshore Nova Scotia (NE Canada) are notable areas ofmore » current oil and gas exploration and production. Significant exploration activity is also targeting areas offshore Angola and Brazil. The extent of deepwater exploration in the GoM is illustrated in Figure 2 that shows the steady march into deeper water, together with a focusing of efforts in the Sigsbee Escarpment areas of Green Canyon, Walker Ridge and Atwater Valley. The deepest wells in the GoM are reaching true vertical depths of up to 32,000 feet, with maximum-recorded downhole pressures in excess of 26,000 psi and bottomhole temperatures in excess of 400 F. Such wells may penetrate considerable thicknesses of salt - up to 20,000 feet of salt is not unheard of. With substantial discoveries and yet-to-find hydrocarbons being overlaid by salt, the impact of this 'mobile' formation on the entrapment of hydrocarbons has received much attention[2]. From a drilling and well integrity standpoint, however, the abundance of salt presents new and significant challenges of a geomechanical nature. Opportunities exist also, as the thick salt sections oftentimes permit the drilling of these deep wells in the first place. During the past five years, the industry has developed a more thorough understanding of salt-related risks. This paper draws upon many of these recent advances to formulate in detail the necessary geomechanical considerations for the successful design of through- and near-salt wells.« less

Improving well design has and always will be the primary goal in drilling operations in the oil and gas industry. To address this issue, an analysis of wellbore stability and well design improvement has been conducted. This study will show a systematic approach to well design by focusing on best practices for mud weight window projection for a field in Mississippi Canyon, Gulf of Mexico. The field includes depleted reservoirs and is in close proximity of salt intrusions. Analysis of offset wells has been conducted in the interest of developing an accurate picture of the subsurface environment by making connections between depth, Non-Productive Time (NPT) events, and mud weights used. Commonly practiced petro physical methods of pore pressure, fracture pressure, and shear failure gradient prediction have been applied to key offset wells in order to enhance the well design for a proposed well. For the first time in the literature, the accuracy of the commonly accepted, seismic interval velocity based and the relatively new, seismic frequency based methodologies for pore pressure prediction are compared. Each of these methods is compared to the petro physically derived mud weight windows for the key offset wells and the proposed well in this field, showing higher reliability in the frequency based approach. Additionally, the interval velocity method yielded erroneous results in a fast-rock-velocity channel zone and the near salt proximity environments, whereas the frequency Based method appeared unaffected by either of these factors.

well. Efficient well construction depends on a comprehensive understanding of the factors that influence the hydraulics of groundwater flow to wells. Considerable field and experimental study has been devoted to the subject. Many investigators have proposed solutions for the flow of groundwater to wells. These solutions, for the most part, have been confirmed to be sufficiently accurate to allow engineers to properly design and managegroundwater resources. In some cases, however, design parameters not fully understood were oversimplified or grossly approximated-resulting in inefficient wells. culated for the six screens based on differences in water levels inside and outside the screen. Testing procedures involved varying the discharge rates and measuring water levels in the aquifer, filter zone, and well. Entrance velocities and sand content were measured for each of the tests. A complete description is given elsewhere.] dh/dr = a$R2/gkd (2) in which R= Reynolds number = ud/v; v = velocity; k = intrinsic permeability; d = characteristic grain diameter (assumed = 50 percent passing size); v = kinematic viscosity of water (1.233 X 10-5 sq ft/s); = constants (aI = 1 for laminar %$ uz = l/R,); and g = gravitational constant (32.2 fpsz [9.8 m/s’]). Much of the information presented in this article is based on results obtained from five years of testing and analysis using this model. Emphasis is placed on those aspects of well design that pertain to wasteful losses of energy and the methods used to control or minimize such losses. The characteristic length in the standard definition of Reynolds number has been replaced by a characteristic grain diameter, assumed equal to the 50 percent passinggrain size. This substitution was made because the microscopic passageways occupied by water in porous media are difficult to measure.

We report high photoresponse measured in a room temperature quantum cascade detector (QCD) based on a coupled quantum well design that operates with a peak response wavelength of 5.4 μm. The coupled quantum well design is expected to produce higher photocurrents when compared with device active regions that use a combination of simple quantum wells. The coupled quantum well QCD demonstrated high responsivity of 22 mA/W at room temperature with a commonly used 45° wedge-based light coupling configuration. Application of a waveguide configuration to the proposed QCD yielded an elevated responsivity of ∼130 mA/W and a specific detectivity (D*) of 1.1 × 108 cm W−1 Hz1/2 at room temperature.

Detailed vertical sampling is useful in aquifers where vertical mixing is limited and steep vertical gradients in chemical concentrations are expected. Samples can be collected at closely spaced vertical intervals from nested wells with short screened intervals. However, this approach may not be appropriate in all situations. An easy-to-construct and easy-to-install multiport sampling well to collect ground-water samples from closely spaced vertical intervals was developed and tested. The multiport sampling well was designed to sample ground water from surficial sand-and-gravel aquifers. The device consists of multiple stainless-steel tubes within a polyvinyl chloride (PVC) protective casing. The tubes protrude through the wall of the PVC casing at the desired sampling depths. A peristaltic pump is used to collect ground-water samples form the sampling ports. The difference in hydraulic head between any two sampling ports can be measured with a vacuum pump and a modified manometer. The usefulness and versatility of this multiport well design was demonstrated at an agricultural research site near Princeton, Minnesota where sampling ports were installed to a maximum depth of about 12 m below land surface. Trace experiments were conducted using potassium bromide to document the degree to which short-circuiting occurred between sampling ports. Samples were successfullymore » collected for analysis of major cations and anions, nutrients, selected herbicides, isotopes, dissolved gases, and chlorofluorocarbon concentrations.« less

Determination of the nature, extent, and rate of off-site chemical migration are common objectives of hazardous waste site investigations. Chemical analyses of water samples from monitoring wells and measurements of hydraulic head and hydraulic conductivity provide the basis for making these determinations. Accurate site assessment, therefore, depends upon the appropriate monitoring well design and sampling and testing procedures. During the course of remedial investigations in Niagara Falls, New York, it has been necessary to evaluate the ground water quality and hydraulic characteristics of 5- to 30-feet thick overburden formations. Many of the monitoring wells completed to these formations consist of a partially penetrating screen (5 feet at the base of the formation) with a fully penetrating sandpack. Questions regarding how this well design influences the source of sampled ground water and hydraulic tests were examined using an extremely fine axisymmetric grid with SATURN, a two-dimensional, finite-element ground water model, and a particle tracking post-processor. A discrete sensitivity analysis was made to determine how flow patterns induced by pumping at 1 gpm are affected by: different screen and sandpack configurations, the ratio of sandpack to formation hydraulic conductivities, heterogeneity, anisotropy, and sandpack thickness. The simulations show that the source (and chemistry given a non-uniform chemical distribution) of ground water sampled will vary considerably depending on a number of factors. Analysis of simulated drawdowns in the monitoring well during purging shows that calculated transmissivities for the range of well designs and conditions modeled will be accurate to within one-half order of magnitude.

Torque and drag analysis has been used for more than ten years to drill wells 'on paper'. Representations of the drillstring and bottomhole assembly (BHA) that do not include the individual component stiffness have been used with some success in analyzing and preventing problems in extended reach drilling. As curve rates have increased in directional wells and the trajectories themselves become more complex, the effects of the previously neglected stiffness have led to errors in the sideforce calculations that underpin all torque and drag analysis. Finite-element analysis can solve this problem. Additionally, tortuosity can be added to provide a rippled effect to a well trajectory design that simulates the micro dog-legs that occur in the drilled well, enabling more realistic bounds to be placed on the drag and torque losses, and the forces experienced by the drillstring. Tortuosity also allows more accurate modeling of the side forces between the casing and drillpipe, critical to extended reach well design. This is illustrated using a well with a deep kick-off point from the Gulf of Mexico. A case study based on wells drilled from the Hibernia platform is used to illustrate how the model has been utilized to aid planning decisions and reduce costs.

Combining the competencies of the well engineer and the geoscientist from the earliest phases of exploration is reaping economic benefit for major capital drilling projects (single wells costing at least US$25 million). As the geologic and geophysical modeling of the explorationist matures and the subsurface picture becomes clearer, appropriate well design flexibility can be achieved using mechanical Earth model (MEM) technology. This approach to well planning is particularly beneficial for deepwater exploration.

Abstract The development of appropriate design procedures for water wells is a subject that has evolved only slowly over the years. Guidelines quoted in most groundwater engineering textbooks are largely empirical and focus on the provision of reliable wells that produce the intended yield with as small a drawdown as possible. The principle of developing designs that minimize the overall cost of the well installation, including the cost of energy used in pumping, is not really addressed. A critical review is presented of established design procedures, and techniques that have been developed for quantifying the energy potentially lost within alternative well designs. Following this review, methods which are more physically realistic and reliable are identified to calculate the key energy loss components of screen upflow losses and partial penetration losses. This, in turn, leads to the development of a way of identifying the costs associated with pumping as a function of the design of the well (most notably the well screen length), which can be used to optimize well design. A comprehensive set of well design guidelines is thereby developed.

Modern Well Design Second Edition presents a unified approach to the well design process and drilling operations. Following an introduction to the field, the second chapter addresses drilling fluids, as well as optimal mud weight, hole cleaning, hydraulic optimization, and methods to handle circulation losses. A relatively large chapter on geomechanics follows, presenting methods for wellbore fracture and collapse modeling, including methods to interpret caliper logs.