Surface-Controlled Subsurface Safety Valves (SCSSVs) are designed to automatically shut-in a well below the earth's surface in the event of damage to the wellhead or other key components of a completion string. Flow rates in highly productive gas wells can produce stresses that exceed the design limits of a typical subsurface safety valve during closure in blowout conditions.High-rate uncontrolled flow conditions can act to prevent a SCSSV from closing or can subject the valve to stresses that the valve cannot withstand. Flow tests conducted to verify valve performance under certain flow conditions are often not capable of predicting valve performance when the valves are subjected to flow conditions different from the test conditions. Because the net force on the flapper is a function of the flow parameters, a valve that tests successfully at one set of conditions is not guaranteed to perform successfully in a different set of conditions. This paper will discuss the use of Finite Element Analysis (FEA) combined with Computational Fluid Dynamics (CFD) to economically and effectively determine 1) if a SCSSV will close under various flow conditions, and 2) whether it will withstand the associated forces generated during the closure.If this method is used, the flow tests could then be used to verify the accuracy and confirm results of the FEA and CFD analyses. After the FEA/CFD models are validated through flow testing, the performance of the valve in response to changes within the flow parameters or safety valve configuration can then be predicted using FEA/CFD models. Actual testing can be performed only to confirm the results of FEA/CFD or may be avoided completely. Introduction Traditionally, SCSSVs are used in well completion strings to permit the controlled production of formation fluids, but in the event of damage or destruction of the wellhead or surface-control equipment, the SCSSV will shut off production flow. SCSSVs are an integral part of the emergency shut-down systems of oil and gas offshore production platforms. In this scenario, the safety valves ensure that uncontrolled wellbore fluids can not reach the surface and contribute to a hazardous condition. SCSSVs are designed to operate in specific environments. Some of the design parameters that are considered when designing a safety valve are: the well operating pressure (initial and depleted), setting depth, operating temperatures, produced-fluid properties and flow rates, corrosive/erosive components in the production and completion fluids, surface-control equipment, umbilical constraints, and tubing- and well-operating envelopes. SCSSVs typically operate at pressures of up to 20,000 psi and in setting depths up to 10,000 ft, depending on the depth of the ocean floor and setting depth below the mud line. Gas wells can have an unconstrained flowrate greater than 450 million standard cubic feet per day (MMscf/D). These high flow rates produce high flow velocities, even when relatively large ID completion strings are used. Safety valves are exposed to fluids ranging from zonal-isolation cements, various weights and solid-content drilling muds, corrosive produced fluids with formation sands to corrosive fracturing fluids. Safety valves must operate in harsh environmental conditions that act to undermine the very function for which the safety valves were designed _ to close off and seal the wellbore in a blowout or potentially explosive event.
Proposal Expandable screens are becoming increasingly more important in sand-control completions because of their exceptional capabilities for providing wellbore support and optimized wellbore geometry.This is particularly important for offshore horizontal wells.Currently, two types of screen expansion technologies exist; i.e., the constant-diameter (fixed-cone) expansion technology and the variable expansion technology.Each approach has its own advantages and disadvantages in terms of deployment, wellbore support, and inflow performance characteristics. With fixed-cone expansion technology, the industry has had the concern that annular flows between the open hole and screen can lead to transport of formation sands. This phenomenon can lead to localized screen erosion due to partial screen blockage, concentrated flow in unblocked portions of the screen, and finally, failure of the filtration layers.This paper discusses a method that can determine the probability of the above concerns with a model that analyzes fluid flow and particle transport.The forces acting on individual sand particles are modeled to ascertain the particle transport within the annulus and determine annular flow problems.Based on a given production profile that takes into account the frictional pressure losses of fluid in the annulus and in the base pipe, this model can predict if there is any possibility that particles in the annulus will be transported.Using this method, the wellbore (annulus) section in which no particles would be transported can be predicted. Computational Fluid Dynamic (CFD) simulations were carried out to study the flow distribution in the horizontal wellbore.At a given wellbore location, the percentages of fluid flowing inside the basepipe and in the annulus were calculated.The simulations typically showed that the average fluid velocity in the annulus is an order of magnitude less than the fluid velocity inside the base pipe.The study also showed that if designed properly, a screen with fixed-cone expansion can be used successfully without concern of particle transport and with very limited flow in the annulus. This paper provides insight into the movement of formation sand in the wellbore annulus.Sand control screen performance can be determined from the models proposed in this study.Results from a field application are presented to demonstrate the application of the model. Background Sand production, which occurs from phenomena such as (1) failure of formation rock due to high stresses resulting from hydraulic or mechanical forces, (2) erosion caused by fluid movement, and (3) chemical reaction due to water production that dislodges some sand particles, is one of the most costly problems in the oilfield.As a result, many different types of sand-control methods have been developed. These include screen designs of different types, gravel pack techniques, fracpacking, and expandable screens, with the choice of which method is used dependant on the type of formation involved. Screens provide sand control by physically blocking sand particles of greater than a certain size by tightly controlling the opening size of the screen.Typically, a natural pack is formed by the larger particles in the annulus between the wellbore and the screen.The gravelpack technique controls sand production by providing a barrier of a sand pack between the screen and a borehole.The particle size distribution in the gravelpack sand dictates the size of the particles that can flow through the pack.Fracpacking uses a mechanism similar to gravelpack, but in addition, a fracture is created to bypass near wellbore damage.The expandable screens function in a manner similar to the stand-alone screens except that annular clearance is reduced. The following sections discuss expansion characteristics.
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