Resistivity is the ability of a substance to impede the flow of an electrical current. This is a very important rock property in formation evaluation as it helps to differentiate between formations filled with salty waters (good conductors of electricity) and those filled with hydrocarbons (poor conductors of electricity). Hence, a difference in resistivity exists between rocks filled with hydrocarbons and those filled with formation water. Resistivity and porosity measurements are used to obtain values of water saturation to help evaluate producibility of the formation.

 

The EWR resistivity sensor belonging to propagation resistivity with one frequency of 2MHZ  measures resistivity using 2MHz frequency(named SFPR) and three compensated spacing. This provides very deep resistivities for bed boundary detection while geosteering. There is a 6 Transmitter and double Receiver symmetric compensation structure, and it could automatically compensated the measuring of temperature and wellbore. It provides the deep, middle and shallow compensated resistivity curves and 6 single transmitter-double receiver resistivity curves. These curves have been characterized for depth of investigation and utilized to generate an accurate interpretation of Rxo an Rt.

Applications

 

Rt, Rxo and Di in invaded formations

Rv and Rh in anisotropic formations

High-resolution Rt in deviated wells and/or dipping beds

Pre-invasion Rt measurement in deeply invading formations

Pre-washout Rt measurement in unconsolidated formations

LWD replacement for wireline array resistivity tools

 

The DGR sensor measures the natural gamma-ray activity of formation. This helps distinguish between reservoir and non-reservoir rocks. It has two measuring sensors conducted by two sets of Geiger-Muller tubes. The sensors are located only 0.55m from the bottom of the instrument, readings are less affected by formation invasion and more quickly get the measuring data.

Applications

 

Correlation

Shale volume calculation

Casing/coring point selection

Geosteering

 

COTS can provide integrated PR-CGR, RR-DGR or PR-AGR service combined with PWD-DDS service. In general, COTS recommend SFPR-DGR-PWD sub connected on base of PMWD or EMWD platform as main LWD service.