Monday, July 31, 2017

Well Testing Design and Analysis chapter 3

The final interpretation step, called history matching or verification, uses the model established in the second buildup to predict presure response throughout all four periods of the test and confirms that the model satisfactorily accounts for all data. This may result in more parameter adjustment because every period must now be matched simulatneously, even though the second flow period is planned intentionally long to minimize the influence of previous periods.

The design phase not only maps out the mechanis of a test, but also ensures that, once underway objectives are met. For example, the progress of the planned transient can be followed at the wellsite and compared with that forecast during the design. To avoid the costly mistake of rigging down before the transient indicates a desired feature, wellsite validation of data during the test remains a must. This is best accomplished with surface readout of downhole gauges and enough computing power at the surface to produce approriate plots, notably the log-log diagnostic plot. If the reservoir response is quite different from the assumed in the design, wellsite diagnosis permits an instant correction of the job, perhaps a lengthening of the transient, to ensure optimum use for the data. In certain cases, real-time readout is not feasible and downhole recording must be used. Data validation can still be performed onsite right after retrieving the gauges. 

Layered reservoir testing (LRT) was originaly conceived to investigate production wells. Recently in offshore Congo, AGIP used the technique to evaluate a layered  reservoir encountered by an exploration well. Conventional testing of individual pay zones in an exploration well would normally call for a separate DST-perforation run for each zone. But using layered reservoir testing, AGIP obtained reliable kh, skin and productivity index values for individual zones with only one trip in the hole, at considerable cost savings. 

The drawback of using an LRT in the exploration setting is that production from different zones commingles, ruling out representative sampling from different pay zones. Fortunately, a recent tecnological provides a solution. Samples of extraordinary reliability may now be obtained from any number of zones using the new wireline-conveyed MDT Modular Formation Dyanmics Tester, but this has to be planned in advance because the sampling takes place in open hole. 

AGIP's innovative use of layered reservoir testing (LRT) in an exploration well occured offshore Congo. The goal of the test was to evaluate two producing layers only a few meters apart with only one trip into the hole, a much less costly undertaking than the usual two trips. The LRT technique was originally developed for testing production wells, in which several layers produce commingled-in this case, LRT is a must to evaluate each layer's dynamic properties. 

source : Oilfield review magazine 

Well Testing Design and Analysis chapter 2

First comes wellbore storage, which refers to the obfuscating role of the wellbore fluid when a transient is initiated. The moment a well is shut in or allowed to flow, fluids in the wellbore must first compress or expand before formation fluids can react. If flow is controlled from the surface, the entire well's fluid contribute to wellbore storage and the effect can dominate the pressure transient for hours afterward. The effect is exacerbated if well pressure toward the top of the well drops below bubblepoint and part of the well is filled with compressible gas. Wellbore storage is substantially reduced by shutting in the well downhole, minimizing the volume of fluids that contribute.

Estimating near-wellbore damage, reduce it if necessary with a matrix acidization and the checking that the acid cleanup worked. This was intended not only to benefit production in the well, but also to help plan a completion strategy for field development. The other goal was to investigate reservoir volume and identifiy reservoir boundaries.

As wellbore storages disspates, the transient begins to move into the formation. Pressure continues building up, but at a slower rate as the transient moves far enough to achieve radial flow toward the wellbore. This is the so called radial-flow regime that appears as a straight line trend on the Horner plot. The radial-flow regime is crucial to quantitative interpretation, since it provides values for kh and skin, S, a measure of the extra pressure drop caused by wellbore damage. Skin takes positive values in a damaged well when pressure drop near the wellbore is greater than expected and negative values when stimulation creates less pressure drop. Next, the transient encounters the limits of the reservoir and pressure departs from its straight-line radial flow response.

Sunday, July 30, 2017

Well Testing Design and Analysis chapter 1

In its simplest form, testing provides short-term production of reservoir fluids to the surface permitting the operator to confirm the show-indicated by cuttings, cores and logs- and estimate reservoir deliverability. In its subtlest form, measured pressure transients caused by abrupt changes in production can characterize completion damage, reservoir permeability and distant reservoir heterogeneities.

Primary concerns in testing exploration wells are obtaining representatives samples and estimating reservoir producibility. Fluid samples are needed to determine various physical parameters required for well test analysis, such as compressibility and viscosity, and for pressure-volume-temperature (PVT) analysis that unlocks how the hydrocarbon phases coexist at different pressures and temperatures.

For oil, a critical PVT parameter is bubblepoint pressure, the pressure above which oil is undersaturated in gas and below which gas within oil stars being released. Maintaining reservoir pressure above bubblepoint is key to successful testing since the principle of transient analysis, described below, holds only if flow in the reservoir remains monophasic. Estimating reservoir producibility requires achieving stable flow rates at several choke sizes and then determining the productivity index from the slope of the flow versus drawdown pressure data.

If well productivity is less than expected, the wellbore damage may be the cause. This is the next concern in testing exploration wells. Estimating the near-well-bore condition to perform necessary remedial action and ultimately to plan a well completion strategy for the field is accomplished from the transient analysis part of a well test.

Transient analysis, however, reaches deeper than just the near-wellbore region. Today, it contributes so much to characterizing the reservoir that engineers increasingly refer to well testing as reservoir testing. Analysis can indicate the likely producing mechanism of the formation-for example, how much production comes from fractures,, how much from intergranular porosity -and it can determine the producing zone's permeability-thickness product, kh. It can see to the limits of the reservoir indicating the probable shape (but not orientation) of the reservoir boundaries and can show whether the primary recovery mechanism is from water or gas-cap support. This information becomes crucial in the appraisal and production stages of field development when engineers combine testing interpretation results with seismic and geologic data to refine their understanding of the reservoir.

Drawdown pressure measurements to track these events practically mirror the buildup response. In fact, transients can be obtained simply by increasing or decreasing the flow rate.

The primary target is the near-wellbore region (picture above). The goal is to assess formation damage and , if necessary, perform stimulation. Test last just an hour or two. In a conventional test conducted to investigate reservoir boundaries, often called a limit test, the transient must be long enough for the pressure disturbance to reach the boundaries and then create a measurable response in the well. How long this takes depends on formation and fluid characteristics. In particular, the lower the formation permeability, the more time is needed-test can continue for days. Longest lasting are interference tests, in which the effect of a transient created in one well is observed in another, yielding information about reservoir transmissivity and storativity. 

The basic data obtained are change in pressure, delta p, versus elapsed time since the transient was initiated, delta t. 

Thursday, July 6, 2017

South China Sea Oil Reservoir Prospects

In the middle of high tense of geopolitic, South China Sea remains highly potential for hydrocarbon reservoir. China has been showing military power to secure South China Sea , which is disputed by Vietnam, Malaysia, Brunei. This essay provides geological & geophysical general approach to understand the prospect of hydrocarbon in South China Sea.

Study area is located in south of Hong Kong

Geologic cross section from North West to South East

The region lies in about 100 m (230 ft) of water in the Pearl River Basin in the South China Sea, some 100 km south of Hong Kong.
Hydrocarbon-bearing sands were discovered in this region in 1986, and a field there now produces 80,000 barrels of oil per day. Two years later, a wildcat (well A) was aimed at a Miocene carbonate reef at the margin of the field and underlying fluvial deltaic sandstones of Lower Miocene age called the Zhuhai. Log and well test data showed that the 30-m (100 ft) thick sandstones were well sorted, porous and oil bearing. The spatial distribution of the sandstones was thought to have been controlled by the overlying reef, which seeemed to act as a caprock. The reef itself was dry, but its extent was uncertain. 
A second well (Well B) was drilled 11 km southeast of the wildcat to explore the lateral extent of the reef. The well encountered a carbonate platform and some reef overlying a thinner Zhuhai sandstone, but indicated poor hydrocarbon shows. 
A third well (Well C) was then drilled on the opposite side of the reef, 3 km northwest of the wildcat, but this also gave dissapointing results despite finding the reef and the sand. 

The interpreters derived an acoustic impedance profile in the wildcat well and the transformed the surface seismic section into an acoustic impedance section over the entire target area. Sands are light green to yellow, while carbonate is blue. It appears that the hydrocarbon-bearing sandstones in Well A pinch out about 350 m to the nortwest, explaining why Well C, over 3000 m away and off the section, tested dry. 

However, the sandstone extends farther southeast but is thought to undergo a facies change, becoming tighter before it reaches Well B, also off the section, as diagenesis increases. The impedance section shows that further drilling should be concentrated close to the back reef area.

(source: Oilfield review , April 1992)