Production logging is the complex downhole logging technique, which is performed with the production logging toolstring. However, modern toolstrings include up to one hundred various sensors, and processing techniques utilize probabilistic non-linear algorithms of multiphase flows, the basics are still the same as 40 years before.
The primary production logging objective is reservoir performance evaluation. In early years, it was known as flowing profile evaluation. The idiom is still the same: it is necessary to estimate what is flowing, where is it flowing and what is the flowing rate. These three answers lead to the final question solution – the major point of interest of the reservoir engineer – why is it flowing (or why is it NOT flowing) under several conditions.
As you can see, production logging requires good knowledge and understanding of reservoir engineering study indeed.
Secondary production logging objectives are lift (or completion) performance evaluation and estimations of factors, affecting the reservoir performance (leaks and crossflows).
Production logging as usually is carried out by the cased hole wireline crew of the service company. That is carried on wells of different definitions: production wells (on different stage of the field development), injection wells (with water, gas or steam stimulation), exploration wells and wildcats (in combination with conventional DST), hydrology wells and steam energy (geothermal) wells. When the well is producer the test is known as production logging test, when the well is injector – the test is known as an injection logging test.
The number and sequence of production logging tests performed on a well-managed field is defined by the field development team. A good practice is to run the Production Logs at an early stage of the life of the well, in order to establish baseline that will be used later when things go wrong. Too often Production Logs are run when something has gone wrong as the last resort (to design well interventions and workovers or even to take decision to abandon the well).
A production logging job starts with PL Survey design. A great amount of data is gathered and analyzed. As much data, as possible should be taken into account: openhole logs, well integrity logs (CBL’s, MultiFinger tools, CASTs, etc), deviation surveys, completion sketches, production (pressures, temperatures, rates) & well intervention history (recent operations). Don't forget the overall geology of the reservoir and detail petrophysical analyssis of the poroisty, permeability, and saturation profiles relative to the existing completion type and location. The main idea is to design safe, cheap and comprehensive Production Logging Test. This will be discussed widely in this chapter.
The result of the PL Survey is the pay zones phase rate profile (total – known as QZT, interval – known as QZI, relative – known as QZTR) and SIP (Selective Inflow Performance diagram).
Selective Inflow performance (SIP) diagram refers to the pay zone producibility index estimation. Being familiar with the total Inflow performance relationship (IPR) diagram which is a part of well testing steady state flow data interpretation and reflects the pressure (or dP) as function of total surface rate, the SIP is constructed for every particular pay zone and flowing phase (even in downhole conditions). SIP may be approximated with linear equation, Vogel, Fetkovich or other inflow relationship (like 2,3 – phase or gas inflow cases). The major purpose of this technique is to evaluate the zone rate with particular pressure difference under several conditions.
If one or several zones, being referred to the same hydraulic system are depleted (due to the higher permeability) the crossflow in shut in between the zones is possible. So, SIP is extremely useful reservoir engineering tool, that provides an opportunity to estimate reservoir pressure, producibility index, possible zone crossflow and depletion for every pay zone and for various fluid phases. To construct the SIP several well flowing regimes (at least two) are required. Well flowing regime means the well is producing with the constant (steady state) or close to steady state at surface. During the PLT the surface multiphase rates are measured as usual and are used later for matching with downhole data and velocity (flowing) model correlation.
Well flowing rate regime is regulated by the choke size (for natural flowing), gaslift injection rate (for gaslift production), ESP power regulation (electric submersible pump case – in this case special completion solution, known as Y-tool, is required, otherwise logging below the pump is not possible), rod pump power regulation (also special completion solution known as “C-type” annulus is required for logging). For injectors, the situation is the same.
The above example illustrates 5 PLT surveys being performed (2 in shut in mode and 3 in flowing). This is not the rule of thumb. Shut in survey (when the well is closed at the surface) is used for downhole tools calibration, pressure estimation and possible crossflow evaluation. This will be covered later in the chapter. In well-known fields the number of flowing regimes may be 2 with no shut in at all, however in exploration wells (or wildcats) I have seen up to 7 flowing regimes with direct and reverse measurements (increasing and decreasing surface rate) and several shut in’s.
In some cases, (low permeability rocks, shale gas formations, etc), the steady state flow cannot be reached (or requires extremely long time for well stabilizing). In this case, the advanced methods, known as isochronal or optimized isochronal tests are being utilized (this is beyonr th4 wcope of production logging chapter, however it may be covered in well testing chapter in future).
The last, but not the least PLT objective is permeability estimation. Yes, in several cases it can be done. To estimate the permeability, refer to Darcy’s law: phase rate, pay thickness, pressure difference, fluid viscosity (from the correlation to oil density) – all these data can be measured and gathered from the PLT Survey. Of course, this is just estimation and should be calibrated with other data sources. For example, the skin-factor (S) can be derived from pressure transient analysis testing technique (pressure build up or draw down), which is also sometimes combined with PLT or some skin assumptions can be done. As you can see, production logging and well testing are in close collaboration, so the knowledge of well testing basics is essential for production logging analysis.
At the end of this brief introduction I would like to mention, that production logging is sometimes combined with well integrity logging (multiarm calipers, ultrasonic thickness devices, or serve itself as an indicator with temperature, flowmeter or noise logging sensors) and cased hole formation evaluation logging (multidetector stationary neutron logs, pulsed neutron logs with spectrometry capability, natural gamma ray spectrometry logs) in through tubing applications.
MORE TO COME when Anton can spare the time from his real job.
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