Sources of errors in production data

The data in both interference and drawdown tests are obtained from pressure gauges.

Although modern gauges often have resolution of±0.01 psi, the accuracy is typically

not that good [43]. Reasons include transient temperature effects and drift in the electronics. Of course the reason for quantifying the magnitude of the errors is that it is pointless to force model to match data to greater accuracy than required by the measurement accuracy. The problem of building a model and matching the data by adjusting parameters depends to a large degree on the accuracy of the assumptions and the approximations in the model. It would be pointless, for example, to attempt to match pressures in the early times of a drawdown or buildup test if the simulator did not correctly model wellbore storage. Similarly, the magnitude of the tidal effects in some interference tests can be as large as the pressure variation induced by the production

10 100 1,000 10,000 100,000 11/12/1996 3/9/1997 7/4/1997 10/29/1997 2/23/1998 6/20/1998 10/15/1998 2/9/1999 6/6/1999 _{10/1/1999 1/26/2000 5/22/2000 9/16/2000 1/11/2001} 5/8/ 2001 9/2/2001 12/28/2001 4/24/2002 8/19/2002 12/14/2002 ESTIMA TED OIL RA TE (BOPD) W A TER RA TE (BWPD) 0 10 20 30 40 50 60 70 80 90 100 B S W (% ) BOPD BPD TOTAL FL ID BS 1100 1300 1500 1700 1900 2100 11/12/1996 11/12/1997 11/12/1998 11/12/1999 11/11/2000 11/11/2001 11/11/2002 Date

Bottomhole pressure at gaug

e depth (psia)

Daily rates Daily BHP

Figure 6.8. Variability of production data.

rate variation. Although the tidal effects are not actually part of the accuracy of the measurement, the neglect of tidal effects in the modeling would make the data very difficult to match to the accuracy of the gauge.

Rate measurements may be obtained from a separator whose accuracy could probably be estimated reasonably well. In may cases, however, a well is actually tested at most once per month, and intermediate rates are estimated from the well-head pressure. It makes little sense to use the accuracy of the instantaneous measurement as the accuracy of the monthly rate, because it is actually the modeling error that is more important than the measurement error in this case.

Figure 6.8 shows daily measurements of rates and daily measurements of bottom- hole pressure for a well in a large fractured carbonate reservoir. For this reservoir, the rates are typically measured weekly, and daily rates are interpolated from the measured rates. The pressure variability is large, but very little is due to actual measurement error; most is a result of operational fluctuations. Unfortunately, it is not uncommon to use actual pressure measurements (obtained only once per month) with average monthly rates in history matching. The two are not compatible and the result can be classified as modeling error.

6.2

Logs and core data

Two types of commonly acquired data are sensitive only to the region of the reservoir in the immediate vicinity of the wellbore. Well-logging is carried out by lowering a probe equipped with sensors into a well. The sensors make measurements that can be used to estimate rock and fluid properties in the near-well region, and to monitor the well construction. The signals are transmitted from the sensors to the surface

through a cable. After processing and interpretation, the logging signals are plotted against measurement depth in log formats. Core measurements are made on samples of rock that have been removed from the reservoir and brought to the surface. In many instances, the interpretations that are obtained from cores and logs are treated as “hard data,” an unfortunate terminology that implies absolute accuracy.

Well-logging measurements can be placed into three major categories: electrical logging, acoustic logging, and radioactivity logging. Electric logging measures the formation resistivity or conductivity and the spontaneous potential in uncased sections of a borehole. Both the brine in the pore space and the water bound to the clay in formation rocks conduct electric current. The measured conductivity reflects the presence of brine in the rock and water bound to clay minerals, hence indicates the probable lithology and water saturation. Formations with very low porosity or whose porosity is occupied by nonconductive oil and gas are identified by high resistivity segments in electric logs.

Acoustic logging tools carry an acoustic source in one end and receivers in the other end. The acoustic waves emitted from the source travel through the near-well formation and are recorded by the receivers. The traveltime of the acoustic waves is recorded against the measurement depth. Continuous measurement of acoustic wave traveltime along the well path provides information about porosity, lithology, mechanical rock properties, and the existence of fractures.

Several types of radioactivity logs measure the natural radioactivity of the formation, while others emit radioactive particles and measure the response from the formation. Natural gamma logging recognizes lithologies and soil contents by measuring the natural radiation emitted from the rock. In general, potassium, uranium, and thorium are the primary contributors to natural radioactivity in the sedimentary formations. These elements primarily occur in clay minerals. The natural gamma ray log simply provides a measurement of the total contribution, while the spectral gamma ray log provides an estimate of the contribution from various isotopes.

Two typical logging tools with artificial radioactive sources are the neutron porosity log and the density log. The density log emits gamma rays and records the intensity of scattered gamma rays at the detector. The log provides an estimate of the density of electrons in the formation. The mass density can be estimated fairly accurately from knowledge of the density of electrons. (A correction must be made for hydrogen.) If the densities of the matrix grains and the fluid in the pore space are known, the porosity of the formation can be estimated. The neutron porosity log releases neutrons from its radioactive source, and records the intensity of the returned gamma ray emitted from hydrogen nuclei after neutrons have been captured. The intensity of the signal is proportional to the amount of hydrogen present. The neutron porosity log tends to respond to zones with high porosity (if the fluid is water or oil) and to zones with high clay content because of the bound water. Radioactivity logs can usually be used in both open and cased hole because the gamma ray has sufficient penetration power to the steel casing and the formation.

The only means for direct measurement of formation rock and fluid properties is through representative core samples that have been brought to the surface for analysis. The set of measurements normally are carried out on core plugs approximately 1 or 1.5 inches in diameter instead of on the whole core, which is typically between 2 and 5 inches in diameter. The basic rock and fluid properties to be measured include a lithologic description, porosity, permeability, grain density, fluid saturation, relative permeabilities, capillary pressure relations, and fluid PVT. Measurements are often made at room temperature and at either atmospheric confining pressure or formation confining pressure, or both.