Gaslift Management in Petroleum Development Oman LLC

Gaslift Management in Petroleum Development Oman LLC

Prepared for the 2003 Fall Gas-Lift Workshop, 21-22 October, Kuala Lumpur

By Walrick van Zandvoord

September 2003

Summary

Production Development Oman LLC (PDO) is the main producer in Oman producing around 700,000 bbl/d of black oil with 200,000 bbl/d on gaslift. This paper summarises gaslift management experience in the Northern Directorate of Petroleum Development Oman LLC (PDO), where 80% of the gaslift production is coming from.

Proper management of complex gaslift systems is time consuming and requires a lot of resources. Taking into account the workload of the production engineers, effectively only 2-3 production engineers were available to manage all the gaslift wells and systems in the Northern Directorate. Considering that gaslift production is around 165,000 bbl/d, this means around 70,000 bbl/d per person. This problem has been recognized and more people are currently put on the job. Furthermore, processes have been put in place to get the maximum out of the gaslifted wells.

Continuous optimisation is attempted through automatic re-distribution of gaslift whenever the compressor capacity changes, which requires continuous and accurate updating of the underlying well models.

Decker's Automatic Validation Tester (AVT) is used for quality assurance of gaslift valves before running them and understanding of failures of pulled gaslift valves has just started. One of the wells has been successfully completed with a wireless gaslift system, which allows operating the unloading and operating valves remotely and wireless from surface.

Poor quality of test data is a significant problem in some areas, but this is being addressed by a dedicated task force, responsible for fixing all the meters. Flowing surveys are used to validate and calibrate test data, which is an absolute necessity to be successful in managing complex gaslift fields.

Individual well improvements are achieved by reviewing each well regularly and taking remedial actions when required. Production improvements have been achieved in a number of surging wells by stabilizing them.

It is estimated that the gaslift efforts are adding around 5% to the gaslift production each year in PDO of which 1% is directly measurable.

Introduction

This paper provides insight in the process of gaslift management in the Northern Directorate of Production Development Oman LLC (PDO) and gives an overview of some of the technical challenges in the different fields spread out over the North of Oman. PDO currently operates some 700,000 bbl/d of black oil production (excluding gas and condensate) of which around 365,000 bbl/d is produced in the Northern Directorate. Most of the production in the Southern Directorate comes from beam pumps and ESP lift, but in the Northern Directorate around 165,000 bbl/d is produced through gaslift. Other production methods are natural flow (40,000 bbl/d) and ESP lift (160,000 bbl/d). Despite a large contribution from ESP lift, gaslift is expected to remain an important lift mechanism.

The following locations are operated by the Northern Directorate (only % gaslift production of the 165,000 bbl/d total gaslift production is mentioned below):

1.  Area D consisting of Field L (33%) and Field D (0.6%).

2.  Area A consisting of Field Y (23%) and Field A (4%).

3.  Area C, comprising Field N (9%) and Field F (24%).

4.  Area B with the majority of the gaslift production coming from Field S (3%) and the rest of the gaslift production coming from Field Q (0.7%), Field G (0.2%), Field B (1%), Field R (0.3%), Field M (currently closed in for flaring and integrity constraints) and Field C (1%).

More detail per area and general information that applies to the entire Northern Directorate is given below.

General Information for all 4 Areas

All gaslift wells in the Northern Directorate are injection controlled. This means control valves are installed which have targets set in the DCS (Distributed Control System) or SCADA (Supervisory Control and Data Acquisition) system. IPO (Injection Pressure Operated) valves are normally used as unloading valves and an orifice valve at the operating point. The only exception is when production is from dual strings, in which case PPO (Production Pressure Operated) valves are used. Currently this is only the case for a handful of wells out of some 500 gaslift wells.

The majority of the gas-lift designs consist of 2 to 5 mandrels, depending on the depth of the packer and the deepest possible lift point during the well's life. Usually the orifice valve is installed in the lowest mandrel, while in the upper mandrels unloading valves are installed with the same port size as the orifice valve. This ensures that the well is always lifting from the deepest mandrel possible, but it is possible that the well continuously lifts through an unloading valve (e.g. when the PI or reservoir pressure is too high to transfer down to the deepest mandrel). Multi-pointing could occur when the injection rate is increased to too high values. It is also possible that the pressure differential between casing and tubing pressure at the lowest injection point not enough, leading to gas injection through the unloading valve. Multi pointing on its own may not be a problem, but when it causes valve cycling, it could lead to well instabilities and it shortens the life of the valves.

The major advantage of the above design is that when reservoir pressure and / or PI changes the well will automatically change its optimal lifting point by transferring gas-lift injection up or down. This also means that individual gaslift well optimisation opportunities are in general very small. Most remedial work on gaslift wells is therefore related to failures or stabilising heading and / or surging wells.

To get the maximum out of gaslift, a gaslift management process has been embedded in the organisation, which attempts to address the following:

1.  Optimally (re-)distribute gaslift.

This is achieved by continuously keeping all well models up to date and regularly create a distribution table against (in most areas) a limited compressor capacity. In some areas (i.e. Area A) the distribution table is calculated at different compressor availabilities. This is then uploaded as a table into the SCADA system, which allows resetting of all the control valves in the DCS system at any time when one or more of the compressors is down. Theoretically this will minimise compressor related deferment. Since in Area A there is sufficient compressor capacity, the gaslift distribution for the case when all compressors are running basically gives each well the maximum gas lift that prevents multi-pointing of the valves. However, in other areas such as Area C and L, where the compressors are full, this is not the case.

The picture below shows the gaslift optimisation system as it is working in most areas in PDO.

Although this may look good, there are a number of technical challenges that make it difficult to always achieve the optimal. Those are listed below:

a.  In general gas measurements are of insufficient accuracy. GOR's are very often over or underestimated by a factor of 2 or more. The gas lift requirement per well strongly depends on its GOR and therefore these errors make it extremely difficult to achieve the optimum. Currently a major effort is ongoing in PDO to improve measurement of all hydrocarbon streams. Also flowing surveys help to calibrate the test data, but those surveys may only be available once per year per well at the most. With the current number of gaslift wells this would already lead to 2 full time W/L units on flowing surveys. Therefore a combination of measurement improvements and regular gaslift surveillance has been chosen as the long-term solution.

b.  In some areas, notably in Area A and L, well integrity is poor. The oldest wells have reached an age of 40 years now. Gaslift surveillance very often shows leaks or holes in the tubing and sometimes even the casing. Symptoms of well integrity problems are severe slugging, low casing pressure, loss of liftgas in the well, flowouts from nearby wells (i.e. when in extreme cases liftgas is injected in shallow aquifers resulting in uncontrolled flow through poor cement bonds in nearby wells, which has happened a few times). Some 40% of the well stock in the Field Y suffers from integrity problems. In Field L there are similar problems, although not as bad. Whenever, the environment or safety is in danger, repairs can easily be justified. However, in some cases a workover cannot be easily justified in view of the low restoration gain of such wells. The repairs, especially when also the casing is involved, may take 2 weeks up to a month depending on the severity (e.g. the amount of fishing that is required and the number and size of the leaks to be fixed). Apart from the fact that well integrity leads to suboptimal well behaviour, also the optimal distribution efforts mentioned under point a) are at risk. The well models assume normally operating gaslift wells. E.g. when there are tubing or casing leaks production engineer is not aware of those, the well model will not properly describe the well. Therefore it is important to recognise such wells and put them on manual control so that the right amount of liftgas can be provided. The increased liftgas requirements of these wells also increase the dependency on the compressors.

c.  Most reservoirs in the North of Oman are subject to large-scale waterfloods and continuous development drilling takes place. This means the reservoir pressure, water cut and productivity of wells continuously changes. With the number of available production engineers responsible for gaslift, it has proved extremely difficult to continuously stay on top of what is happening. Currently this is addressed by putting additional people (surveillance engineers) on the problem. More information can be found in the section about Area A.

d.  Most reservoirs are carbonate reservoirs, very often heavily fractured. The dynamics of the reservoir is not modeled in the gaslift well models. This needs to be realised when using the models for optimisation of short-term changes.

2.  Regular gas-lift surveillance

Gaslift wells are regularly reviewed based on the available SCADA data and flowing surveys. Models of each well are maintained and when the analysis is completed the results are stored for later reference, even when no action is taken. This ensures that analysis work is not lost.

When action is taken and gaslift valves are pulled and a new string is re-run, Decker's Automatic Validation Tester (AVT) is used as a quality assurance measure. The AVT tester allows gaslift valves to be rigorously tested. Parameters such as Test Rack Opening Pressure (TROP), Stem travel, R ratio etc. are accurately determined by the AVT tester. Pulled valves are tested to improve understanding what was wrong with them, while new valves are tested to assure quality valves are run into the well. PDO has just started using the AVT tester and the feedback on pulled valves is just starting to funnel through the organization. Initial indications are that many valves fail due to bellows failure or leak. The fact that many of the valves are pulled, because they are suspected to leak, confirms this finding.

Since introduction of the AVT tester in PDO >25% of the valves set in the workshop were rejected, which if run into wells, could have easily led to underperformance of wells. The majority of the reasons for rejection (>90%) is that the AVT measured TROP is not fitting the design TROP within the agreed limits of 15 PSI. Before the AVT tester was used there were even occasions where new wells could not be kicked off, because the TROP was set too high, leading to unnecessary deferment. Currently PDO is working on automatically linking the results of the AVT tester to the gaslift analysis, design and trouble-shooting software WinGLUE so that the actually tested valve performance can be used in the analysis.

3.  Continuous monitoring of gaslift wells

The SCADA system, where almost each well has continuous pressure and injection rate information available, is heavily used to monitor gaslift wells on a day-to-day basis. This is done by dedicated well analysts, who also look after the ESP and naturally flowing wells. In each shift there are 6 well analysts available for the 4 areas (1 1/2 on average per area). Whenever anomalies are found more investigation is carried out and if necessary action is taken.

4.  Semi-automated upload of gaslift well data

Recently a semi-automated link was build from PDO's corporate databases to the gaslift analysis, design and trouble-shooting software WinGLUE. When users now start looking at a well all recent completion, deviation, test and reservoir data can be automatically uploaded on request. This is expected to lead to significant time savings. Initial results have also revealed a number of data quality issues in PDO’s completion database that need to be addressed (e.g. information on choke sizes is missing from the database).

More Specific Information per Area

Area D

Gaslift production in Area D is constrained by flaring restrictions caused by limited compressor capacity. Realising that this is an issue and given the plans for a major infill campaign in the area, a workshop was organised in September 2002 to identify opportunities to optimise the gaslift system and possibly create more space in the compressors. Following up on the workshop the following was achieved:

1.  Around 3,600,000 cft/d of gaslift was saved by reducing gaslift in wells, which has resulted in a reduction of flaring and more available space in the compressors for the new well infill campaign.