Casing Annulus Research Articles

Simulator Optimizes Gas Lift Unloading

This article is a synopsis of paper SPE 29450, "Gas-Lift Unloading and Operating Simulation as Applied to Mandrel Spacing and Valve Design" by James W. Hall, SPE, Producing Solutions and Kenneth L. Decker, SPE, Decker Technology, originally presented at the 1995 SPE Production Operations Symposium, Oklahoma City, OK April 2-4. Please read the full-length paper for additional detail, illustrations, and references. Gas lift has been practiced for many years with principles developed on experience and "rules of thumb" with little regard for actual valve performance. Available software now simulates the unloading and operation of gas lift wells by use of scientific principles and valid "valve-performance" data. In a conventional configuration, the tubing/ casing annulus is initially filled with a completion fluid from the packer to the surface. To initiate gas lift, the fluid must be removed from the annulus to uncover the installed flow-control device at the desired point of gas injection into the tubing. This process of removing the completion fluid is referred to as "unloading" the annulus. Various procedures for unloading the annulus have been recommended. When recommending changes of injection rate and pressure, these procedures ignore the actual performance of the gas lift valve under downhole conditions. For this paper, valve performance data are defined as the quantitative measure of a valve's flow-rate response to changes in casing and/or tubing pressure for a given pressure and temperature setting. Vendors supply tables and figures to predict gas-flow rates for their equipment. The information is presented in different formats and no algorithm is available to compare valve performance of different manufacturers. Additionally, the data are focused on gas-flow, making liquid-flow predictions difficult. Therefore, the unloading process cannot be optimized with these data. API RP 11 V2 outlines a procedure for obtaining performance data of gas lift valves. The procedure is not required and must be requested by the end user. Tests are being performed and results are available. Computer applications applying the valve-performance data have been developed. Another error in some applications is the assumption that gas lift valves are fully open immediately after opening and before closing. This is true only for pilot and differential- pressure-operated valves. The application of valve-performance data to engineering solutions is still not a standard practice. Tools are available to design annulus-unloading procedures for the unique parameters for each well. Prudent operators will be able to examine mandrel spacing and valve design before installation to ensure proper performance, to reduce valve damage, and to eliminate multipoint injection.

Offshore Disposal of Oil-Based Drilling-Fluid Wastes An Environmentally Acceptable Solution

Summary Oily cuttings and waste fluid are byproducts of oil-based drilling muds. In such difficult drilling environments as the Gulf of Mexico, where oil-based fluids often are preferred, personnel safety, environmental, and economic concerns are exacerbated by the necessity to transport these cuttings and fluids to shore for disposal. This paper describes a process for on-site preparation and subsequent disposal of a slurry of cuttings by annular pumping. The disposal includes all cuttings and waste oil mud generated during drilling with oil-based fluids. Wastes are displaced down a casing annulus and into permeable zones below the surface casing setting depth. Descriptions of environmental and safety problems arising from onshore disposal, benefits of annular pumping, and equipment used for slurry preparation and pumping are described. This technique eliminates the need for platform cuttings storage, cuttings transportation to shore, and the environmental effects of onshore disposal.

Detection Of Gas Migration Behind Casing Using Ultrasonic Imaging Methods

Abstract Failure of the cement, providing zonal hydraulic isolation on the outside of the production casing, occurs in some wells and can result in upward migration of gas behind the casing. Such a cement failure might result in a gas flow from the surface vent connected to the production by surface casing annulus. Or, if the vent is closed, the gas might build sufficient pressure at surface to break down the formation strength at the casing shoe and thereby migrate up around the wellhead itself. Prior to attempting repairs, both the location of the gas source and the potential to displace the annular space with cement must be evaluated. These may be determined from an ultrasonic image obtained from the Cement Evaluation Tool (CET*). Temperature and audio logs attempt to detect the actual movement of the gas flow which may be very small and difficult to detect. In such cases the CET image has shown a significant presence of gas filled channels from the source zone up to the surface. Log examples pinpoint probable gas sources and identify annular materials enabling successful remedial cementing strategies to be developed. Introduction The Problem of Surface Gas Migration There are a significant number of wells today leaking gas to surface. In Alberta, Energy Resources Conservation Board statistics indicate that, in 1988,4.7% of wells in Alberta had surface casing vent flows compared to 2.6% in 1931(1). Potential environmental effects include ground water contamination and release of hydrocarbon vapours to the atmosphere. How much gas is a serious problem? What is the cause of the leak? Can it be repaired? How? When should it be repaired? immediately or if not serious, at time of abandonment? Many of these questions are being evaluated by regulatory agencies as well as energy companies. As we enter the 1990s, the onus will be on energy companies to reduce the number of wells that have surface gas leaks. Data is needed to evaluate these problem wells and develop remedial action plans. Part of the data required can be obtained through wireline logging techniques. These measurement techniques have had a measure of success detecting movement near the completed interval in a well. Figure 1 illustrates subsurface gas migration resulting in a surface flow at the casing vent valve. The Gas Migration Advisor (GMA) is a new wireline logging technique that employs ultrasonic imaging to identify the contents of the casing annulus as gas liquid or solid. Quantifying and Measuring the Problem Historically, gas leak detection has been accomplished with varying degrees of success by logging with noise and temperature tools(2). The noise tool responds to audio signals. Detectable signals are caused by fluid or gas moving turbulently through a sufficient pressure differential. By examining the tool response at different locations in the well, significant movement behind casing can be detected leading to an identification of the source of the gas (Fig, 2). The temperature tool measures the temperature in the wellbore.

WELL TUBULAR CORROSION IN THE COOPER/EROMANGA BASIN

In September 1987 the Della-1 gas well blew out at approximately 19.5m (64ft) abovesea level (42.7m (140 ft) KB) due to corrosion of the production casing and tubing.The production casing failure and other similar corrosion occurrences were considered to be due to sulphate-reducing bacteria which have been identified in a large number of wells in the Cooper Basin. It was considered possible that iron sulphide was being deposited on the casings in the surface-to-production casing annulus at the air/water interface promoting the formation of anodic sites and therefore corrosion.Further investigations of the evidence indicates that sulphate-reducing bacteria are not the major contributors to the corrosion as was initially believed. Field studies, laboratory analysis and ongoing well programs show that the process of differential aeration is the prime cause of the casing corrosion. Corrosion has been found to occur predominantly at a depth of between 18.3m (60 ft) and 36.6m (120 ft) above sea level and occurs over a band of 6.1 m (20 ft) to 9.1m (SO ft) in each well in conjunction with the external water table.As a result of this corrosion failure SANTOS has initiated a regular program of well maintenance, annulus inhibitor top-ups and pressure testing. A total of 315 wells have been tested to date, production casing corrosion problems have been identified in 35 wells, 31 wells have been repaired and four wells abandoned.

WASTE INJECTION: TESTING FOR MECHANICAL INTEGRITY1

ABSTRACT: Results of computer calculations are presented which predict the pressure response of a closed casing annulus of an injection well to temperature and pressure variations of the injected waste stream. For example, a 3000 ft. disposal well receiving fluid at 100d̀F might cause an increase in annular pressure of some 500 psi. Graphs summarizing these results are included. The results will be of interest to waste injection well operators, inasmuch as they are required to monitor annulus pressure to prove mechanical integrity of their injection wells.

The Vertical Multiphase Flow of Oil and Gas At High Rates

A method is presented for calculating the pressure drop in a vertical well flowing oil and gas. It is designed primarily for flow rates in excess of 5,000 BID in large-diameter tubing, casing, or casing annuli, and relies heavily on the use of PVT data. Introduction While many investigators have attempted to develop methods for predicting the pressure loss that occurs in a vertical wellbore in which oil and gas are flowing simultaneously, few have specifically considered casing annulus flow, and then only at relatively low rates. Most Middle East oil companies have studied the problem, but little has been published. High-rate wells, problem, but little has been published. High-rate wells, although few in number when compared with the total number of producing wells worldwide, account for a considerable proportion of the world's production; yet none of the presently published articles predicts the two-phase pressure drop with any precision. Sanchez has calculated the bottom-hole flowing pressure for many wells using the methods of Hagedorn and Brown, Duns and Ros, Orkiszewski, and Baxendell and Thomas. Included in Sanchez data are the first 10 wells listed in Table 1. Table 2 compares his results for these 10 wells with those found below. The objective of this paper is to describe a more accurate means for calculating the two-phase pressure drop in a vertical wellbore for flow rates in excess of about 5,000 B/D, with large-diameter tubing, casing, or casing annulus "oil strings." The limitations to the range of application are discussed. Pressure Traverse Equation Pressure Traverse Equation The pressure traverse equation, as used in this paper, is of the form (1) This equation is similar in many ways to that of Poettmann and Carpenter and Hagedorn and Brown Poettmann and Carpenter and Hagedorn and Brown and can be derived from the general energy equation. The fundamental difference between this paper and the work of those previously mentioned is that no general correlations are made from masses of field or experimental data. The pressure traverse is evaluated using PVT data for the crude oil being considered, friction PVT data for the crude oil being considered, friction factors obtained from a standard Moody diagram or the Colebrook equation, and implicitly assumes no slippage between phases. The individual parameters and their treatment are discussed in the following sections. Fluid Density The fluid density is derived from PVT data for the flowing crude oil. These data are usually obtained from laboratory measurements made on a bottom-hole sample collected from the well and pressure sealed during transportation from the field. JPT P. 825

Use of the Automatic Free Piston in Oil Well Production Problems

Introduction Before discussing the use of the free piston, it may be well to explainbriefly the type of gas lift valves with which the tool is designed tofunction. The gas lift valve is nothing more or less than a small back pressureregulator. Its construction consists of an outer housing covering a seamlessmetallic bellows in such a way as to form a sealed pressure vessel or pressuredome. To the bellows is attached a stem, and to this stem is attached a seatingmember. When the valve is run into the hole, a pre-determined pressure chargeis placed in the pressure dome. As shown in Fig. 1 the valve is furnished infour different major assemblies, to accomplish good results in different typesof wells. The valve is mounted on a pup joint (Fig. 2-a); the complete assembly isentered into the tubing string at the desired location. Each valve is furnishedwith a reverse flow check valve (Fig. 2-b) to eliminate any back wash throughthe mechanism. Valve action is as follows: At any time the casing pressure is allowed to exceed the pressure forceswithin the pressure dome of the valve, the valve simply opens and passes gasfrom the casing annulus into the tubing. When the casing pressure forces areallowed to bleed to a force less than the pressure force within the dome of thevalve, the valve closes. With this type of construction, it is possible to open and close the valveor valves at will from the surface, through the medium of the gas pressure inthe casing annulus. The cycle of operation is governed by a time cycle motorvalve, commonly referred to as a gas lift intermitter or gas life surfacecontroller (Fig. 3). In actual practice, after the well is unloaded only onevalve is in operation… all other valves remain in the closed position. Onpre-determined time cycles, it is possible to open and close this operating valve at will, and to pass thedesired amount of gas through this valve whenever needed. This method of gas lift is widely used and has been found to be veryefficient. However, it has long been known that even greater efficiency wouldbe obtained if it were possible to place a piston between the injected gas andthe fluid head. The free piston which will be discussed here was designed toaccomplish this end and has been found to do so with very satisfactoryresults. T.P. 3583