Production Logging Definition

Schlumberger

D . DEFINITION OF PRODUCTION LOGGING D . 1 DEFINITION OF PRODUCTION LOGGING Production logging is the measurement of fluid parameters on a zone-by-zone basis to yield information about the type and movement of fluids within and near the wellbore. Production logging is intended primarily for measuring the performance of producing wells. It provides diagnostic information, pinpoints where fluids such as water, oil and gas are entering a well and gives an indication about the efficiency of the perforations. Traditional production logging involves four measurements - flow, density, temperature and pressure. However, only the flow and density readings are used in traditional quantitative production logging analysis. Temperature and pressure data have normally been used in a qualitative way to compute in-situ flow properties and locate zones of entry of fluid into a well. D . 2 HISTORY OF PRODUCTION LOGGING Modern Production Logging is far from the early beginnings of the technique, with highly accurate sensors all on a single tool with simultaneous acquisition. However a lot of sensors go back some considerable time. Temperature surveys were first used in the mid 1930s. One use was the estimation of the top of the cement behind the casing. The setting process of the cement is an exothermic reaction, it gives off heat. Hence the temperature sensor “sees” where there is cement in the well. (Note; this method is still used, in order to work well the log has to be run less than 12 hours after the cement has been pumped.) By the late 1950s and early 1960s the basic sensor types had been developed as individual tools. The surveys required a seperate pass to obtain flowmeter, gradiomanometer, temperature and so on. 1970 saw the sensors packaged together in one tool, meaning a more efficient

single run in the hole. The individual measurements still had to be run one at a time. By the end of the decade advances in electronics allowed everything to be recorded in a single pass across the zone of interest. This had many advantages not least the savings in time. Improvements continued through the 1980s to the present day with better sensors, especially pressure gauges, and deployment methods. The latest tool uses completely new technology to measure a flow profile for the individual fluid phases all around the borehole. D . 3 USES OF PRODUCTION LOGGING Production Logging is put to many uses depending on the reservoir type, well conditions and the perceived problem. (See Figure D1). Some of the major ones are: 1. Evaluate completion performance - New wells - Injection wells - Re-completions 2. Monitor reservoir performance & variations - Flow profile - Well test - Completion Efficiency 3. Diagnose well problems - Water entry - Gas entry - Leaks and mechanical problems - Flow behind casing 4. Other - Guidance for workover - Information for enhanced oil recovery projects - Identify boundaries for field development

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Introduction to Production Logging

Casing Leak Tubing Leak Packer Leak

P1

oil

P2>>P1 Bad Cement

Unwanted fluid flow

P2

Fig. D1: Common problems encountered in the producing wells. Some are due to mechanical problems others to the reservoirs

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Schlumberger

D . 4 PRODUCTION LOGGING MEASUREMENTS D . 4 . 1 Tools Production logging tools consist of a number of sensors which make the measurements inside the well (Figure D2). The main types are: 1. Flowrate (fluid velocity) measurement - Spinner rotation 2. Fluid density measurement - Differential pressure - Gamma ray attentuation 3. Well bore temperature - Variance in resistance 4. Well bore pressure - Strain gauges - Crystal gauges A number of auxiliary measurements are used to augment or assist in the analysis of the major logs. They are: - GR /CCL for correlation - Caliper (mechanical) - Fluid sampling - Noise Logs - Tracer surveys - Water Flow Log

Flowmeter

Fig. D2: A typical production logging tool string contains a number of sensors

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Introduction to Production Logging

1. Flowmeter - Determine producing zones - Stimulation evaluation - Secondary recovery - Flow potential evaluation (SIP, AOF) 2. Temperature - Location of production or injection zones - Monitor frac performance - Gas entry - Fluid movement behind pipe - Fluid conversions 3. Fluid Density - Determine volumetric flow in two phase flow - Show entry points in three phase flow 4. Pressure - Well test analysis (kh, skin) - Reservoir extent, boundaries - Fluid conversions - AOF, SIP determination

Up Run

Gradio

600

Perforations

D . 4 . 2 Applications of specific measurements Each sensor has some specific uses, most are utilised in combination, however, to give a total answer for the well/reservoir.

Down Run

Temperature

Spinners

700

800

Fig. D3: A typical production log.

D . 5 PRODUCTION LOGGING ENVIRONMENT The production logging environment is very different from that of open hole logging. Firstly in place there is normally a completion, which can take many forms. The reservoir zone may be open hole, perforated casing or gravel pack. There may be single or multiple zones and single or multiple tubings. The log is normally run in dynamic conditions, the well is flowing mixtures of liquids and gases. - Oil, water, polymers. - Methane +, N2, CO2, H2S, He. (Quite often there are solids present - formation, frac propant, paraffin, scale, diverter balls, etc.) Hence care and attention has to be taken in the logging program so that the maximum information is obtained to answer the problem.

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Schlumberger

D . 6 LOGGING AND INTERPRETATION PROCEDURES The procedure to ensure a successful production log is simple and can be broken down into three steps, - programming the job, - running the job and - interpreting the data. D . 6 . 1 Programming the job The first step starts with defining the problem: e.g. Oil production is falling, water cut is increasing. Then list and quantify symptoms and well conditions, for example: • Water Cut has increased from 2% to 15% in six months • Total production has fallen from 800 to 500 B/D • GOR - 350 cu ft/bbl • Tubing head pressure - 1200 psia • Oil gravity - 30 oAPI • Gas gravity - 0.7

Then determine if there is a reasonable possibility of solving the problem with available sensors. For instance: • The well is producing above the bubble point (down hole) and downhole water production is greater than 10% of the total downhole flow. D . 6 . 2 Running the job The second step starts with gathering all the required data. • Calibrate the tools • Maintain depth control • Record data optically and magnetically D . 6 . 3 Interpreting the data Choose a Single or Biphasic interpretation model. Select Computer interpretation or manual. In both cases the general equations are the same. Qh = Yh Qt - Yh (1 - Yh) Vs A Ql = Qt - Qh Qt - Total flowrate

Then define sensors needed and technique necessary to gather required data. (Mechanical configuration of the well must be considered.) This may include: • Fluid velocity, density, pressure, and temperature need to be measured • Data is to be taken vs depth and vs time with the well flowing and static • 5 1/2-in. casing set to 9550 ft. 0˚ deviation • 2 7/8-in. tubing set to 9350 ft. • Perforations - 9400-9450 / 9460-9475 • Fill (TD ?)

Qh - Heavy phase flowrate Ql - Light phase flowrate Yh - Heavy phase holdup (decimal percent by volume) Vs - Velocity of the light phase relative to the heavy phase A - Cross-sectional area Finally produce the answer (see Figure D4).

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Introduction to Production Logging

Fig. D4: The result of a production log interpretation

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Schlumberger

D . 7 PRODUCTION LOGGING OPERATIONS Production logging sensors are available in many configurations depending on their intended use. Communication between the operating company and the service company is very important for successful production logging. In any form of well servicing, good communications are wise; but, in production logging, good dialogue is critical to solving production problems. It is also essential for acquiring good base data to more accurately monitor well performance and to solve future production problems. Although there are many types of sensors, this does not necessarily imply that several trips into the well will be needed to solve a particular problem. Acquisition of the various forms of data can often be accomplished with one trip into the well by multiplexing the signals from the combined tool string. In addition to rig time savings and convenience, the reduced number of trips into the hole can produce less disturbance of the production profile as a result of fewer pressure releases with the surface pressure control equipment; this helps assure that all the sensors are logging the flow conditions with simultaneous measurements.

Gamma ray plus collar log will be on depth with the openhole logs.

Casing Collar Locator

Gamma Ray

Flowmeter

D.7.1 Depth Control Casing Collar Locator Section Figure D5 shows a combination tool. The tool has several production logging sensors and a casing collar locator section. As with most tools run in casing, it is very important that casing collars be recorded. Collars are the only positive depth control link between the production logging sensors and the formation strata. Gamma Ray Log The other half of depth control is a gamma ray log run in casing simultaneously with a casing collar log. The gamma ray in casing is depth matched to the openhole logs; therefore, the casing collars that were recorded simultaneously will be on depth, or correctly depth matched, relative to the openhole logs. Any subsequent services run in casing with a casing collar locator that is depth-matched to the

Fig. D5: Standard tool string showing the casing collar locator and gamma ray

This procedure is necessary for the depth measurement accuracy required for perforating, plugs, packers, etc. If cement evaluation is run, a gamma ray and collar locator are usually combined with the cement evaluation tool, typically a sonic device, to acquire depth control data simultaneously with cement information. These logs are not absolutely essential if the production logging tool string contains a gamma ray section; however, the gamma ray collar log is usually run for perforating accuracy far in advance of the decision to run production logging tools that may contain a gamma ray.

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