Test or not to test

8/7/2019 Test or not to test

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Testing for Volume in the Deepwater EnvironmentTalk from the Seat

Richard Molloy

2005 WTN Multi-topic Meeting (MTM#2)


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Questions

Why test for volume?

What are the testing options and the issues with each

(cost, operational, environmental, analysis uncertainty)? Are there any misconceptions about volume testing thatguide operator / industry opinion?

Are we doing this enough? Too much? Not enough? In

appropriate circumstances? What design approach (flow rate schedule, equipment) canwe use to mitigate risks?

Should we treat this differently in the HPHT environment?


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Why Test for Volume?

Commerciality questions must be answered at an early stage inprospect evaluation

A low-side economic scenario must be de-risked eitherreservoir-wide or by well drainage area

Poor seismic data or uncertainty in interpretation leads toquestions about reservoir extent or presence

Reservoir connectivity questions exist due to faultpresence

Would we use well tests or some other means

to answer these questions?


4/15

Reservoir Limits Testing

Test duration sufficient to investigate 2x distance to farthest boundary from well.

Once reservoir limits are reached, P.S.S. pressure regime begins. Flowing pressuresdecline linearly, shut-in pressure approach average reservoir pressure (buildup derivativefalls to zero)

Volume estimated using material balance approach (well documented in well testliterature)

Derivative approaches 0 as shut-in pressure stabilizes.


5/15

LS kh

ML kh

HS khIncreasing kh

More symmetric

drainage area

Thin-width channel

drainage area

Effect of Drainage Area Geometry and kh on Well Test Flow Time

Geometry of drainage area and location of well have large impact on test duration.

Design must be conservative due to uncertainty in stratigraphy within drainage area.

Limits tests in deepwater can be extremely expensive at $500K/day rig rates.

Cost Increasing

distance to fourth

boundary


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Minimum Connected Volume Approach

Pressure history match is generated with an open homogeneous model(typically analytic) with no-flow boundaries.

Fourth boundary is added to model until history match is degraded.

Volume is calculated based on dimensions of analytic model (typically

simple geometries) .

1.57*V

V

1.10*V

1.03*V

Time (hours)

V

0.87*V

1.01*V1.03*V1.10*V1.57*V

Pressure

and

derivative

(psi)

Time (hours)


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Minimum Connected Volume Approach Questions

What are the criteria for selecting afourth boundary distance that does notgrossly over- or under-estimate thetested volume?

History match degradation based on

pressure or derivative criteria?

Can this approach be used with simple analyticmodels to assess connected volume in complexheterogeneous reservoirs (turbidite systems) withconfidence?

What is the impact of aquifer or gas capinfluence on the uncertainty in this approach? Doesthis generally lead to over- or under-estimation of

volumes? How is uncertainty increased if strong tidaleffects are present?

What is the confidence in this technique? (Hint: there arefar fewer publications referencing this technique in well testliterature than limits testing)


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Dual- Flow / Dual- Shutin Well Test - Material Balance Approach

Objective

Confirm a minimum connected pore volume thatrepresents a threshold economic limit.

Approach

Flow well using two periods of roughly equal durationseparated by buildups (Dual-flow / Dual shutin).

Produce sufficient fluid during second flow period to

create a 5 psi depletion in a reservoir of a given threshold(commercial) volume.

Measure pressure at identical times during successivebuildups and determine offset.

Compare measured pressure offset to value calculatedfrom material balance to determine if threshold volumeachieved.

If Pmeas>PMBAL, threshold volume exceeded. If Pmeas>PMBAL, continue buildup or calculateminimum volume.

Q = rate [STBPD]tp = flow time [days]P = measured pressure depletion [psi]

Bo = oil formation volume factor [RB/STB]ct = total compressibility [1/psi]

PV = pore volume [RB]

[PV / Bo] ~ Q * tpP * ct

Material Balance Calculation Pmeas < PMBAL?


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Effect of Parameter Uncertainty on Test Design

Q = rate [STBPD]

tp = flow time [days]P = measured pressure depletion [psi]Bo = oil formation volume factor [RB/STB]

ct = total compressibility [1/psi]PB = pore volume [RB]

[PV / Bo] ~Q * tp

P * ct

Material Balance Calculation


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Well Test Equipment Necessities


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Misconceptions or Facts?

Tests for reservoir volume are too long and expensive indeepwater

Most tests target a single interval, and results are notrepresentative in multi-layered reservoirs

Well test data interpretation has too much uncertainty / non-uniqueness to be used for volume estimation

In deepwater and HPHT, connected volume testing is too riskyoperationally

Volume information can be obtained reasonably well by othermeans

Other Issues

Environmental concerns how do we handle the produced volumes?

Influence of Regulatory agencies flaring or barging issues

Operational risks in HPHT


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BACKUP


13/15

Flowing operations represent an average of 21% of total test time. Avg. total test time was 14.4 d, avg. NPTwas 16%.

Typical test flowing operations includes initial flow and buildup, cleanup and 24 hour main flow, 24 hourbuildup, sampling and max rate flow (avg 3.1 d).

Non-flowing operations include RIH w/ tubing and displacing well to brine, correlating and setting sumppacker, perforate, gravel pack, RIH w/ DST tools, killing well, and POOH with DST assembly (avg 11.3 d).


14/15

Time to investigate a 60 MM RB PV is shown above for various aspect ratio rectangular systems having 50 ft of net thickness, a perm of 1000 md, total

compressibility of 20x10-6 1/psi, and viscosity () of 1.2 cp using the equation below. The constant depends on the aspect ratio (W/L), and the ratio of theboundary distances from the well along the long-axis of the rectangle (Fl).

t (hrs) = PV * ct * / (kh) * Constant

The equation is based on a simple radius of investigation approach which says you must flow the well twice the boundary distance to fully characterize aboundary. The equation calculates times that are conservative because full investigation of the entire reservoir volume is assumed, or stated another way, the

buildup log-log derivative would show an unambiguous indication of reservoir depletion (rollover of the derivative).

The results show that required test times increase as 1) the well moves more toward the left boundary (L2 and Fl approach 0), and 2) the width of the channel

gets small. The explanation in each case is that a 60 MM RB PV is assumed in every case, so for longer rectangles, the transient must move a much greaterdistance to see the farthest boundary. When the well is in the center of a rectangle (Fl=1, W/L = 1), all the boundary distances are equi-distant from the well so

the required test time (based on distance to farthest boundary) is minimized.

Note that the test duration is reduced by a factor of 4 if the test interval thickness (h) and the permeability (k) are doubled to 100 ft and 2000 md, respectively.


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Single FaultRectangle - 150 MBOIPRectangle - 100 MBOIP End of Buildup Data

Radial Flow

Impact of tides

Tide effects add noise to pressure and derivative signal.

Impact is greatest on derivative, which increases uncertainty in model selection.

A practical limit in rock and

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