Page 147 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

116 Reservoir Engineering

(rat contind from page 108)

Introduction

This section deals with the part of formation evaluation known, as well
logging. Well logs are a record versus depth of some physical parameter of the
formation. Parameters such as electrical resistance, naturally occurring radio-
activity, or hydrogen content may be measured so that important producing
characteristics such as porosity, water saturation, pay thickness, and lithology
may be determined. Logging instruments (called sondes) are lowered down the
borehole on armored electrical cable (called a wireline). Readings are taken while
the tool is being raised up the hole. The information is transmitted uphole via
the cable where it is processed by an on-board computer and recorded on
magnetic tape and photographic film. In older logging units, downhole signals
are processed by analog circuits before being recorded.
Well logging can be divided into two areas: open hole and cased hole. Open
hole logging is done after drilling, before casing is set. The purpose of open
hole logging is to evaluate all strata penetrated for the presence of oil and gas.
Open hole logs give more reliable information on producing characteristics than
cased hole logs. Cased hole logs provide information about cement job quality,
casing corrosion, fluid flow characteristics, and reservoir performance. In areas
where the geologic and producing characteristics of a reservoir are well known,
as in development wells, cased hole logs are used for correlation. In recent years,
many new open- and cased-hole logs and services have become available,
including fluid samplers, sidewall cores, fracture height log, and seismic services.
These products, in conjunction with new computer processing techniques,
provide the engineer and geologist with an enormous amount of data for any well.
Parameters that Can Be Calculated or Estimated from Logs
Por~slty. Porosity is defined as the ratio of volume of pores to the total volume
of the rock. It occurs as primary (depositional) or secondary (diagenetic or
solution) porosity. Primary and secondary porosity can be read directly from
neutron, density, and sonic logs. These tools do not measure pore volume
directly, rather they measure physical parameters of the formation and relate
them to porosity mathematically or empirically. Since the sonic tool only records
primary (or matrix) porosity, it can be combined with total porosity tools, such
as density or a combined neutron and density, to determine secondary porosity:

(5-83)
where Qseeondary = porosity due to vugs and fractures
= total porosity as determined from cores, density log, neutron-
density crossplot, or local knowledge
Qsonic = porosity determined from sonic log.
No distinction between effective and total porosity can be made with present
logging methods.
Water Saturation. Connate water saturation (Sw) and flushed zone water
saturation (Sxo) can be calculated from information supplied by well logs.