Using Cf-252 as a Short Half-Life Neutron Source Provides a Safe and Reliable Porosity Alternative When Drilling in a Middle East Carbonate Reservoir

Abstract

UAE environmental regulations call for minimizing exposure to radioactive waste while drilling. In depleted carbonate reservoirs, the low radiation energy, short half-life Californium-252 (Cf-252) neutron source provides a "fit for purpose" replacement to the high radiation energy, long half-life americium-beryllium (Am-Be) neutron source. With the much shorter half-life time Cf-252 neutron source, ADCO could minimize the associated liability of getting stuck downhole by eliminating the use of high energy radioactive sources with 430 year half-lives.

The Cf-252 sources emit neutrons as a result of spontaneous fission with a probability of about 3%. One gram of Cf-252 yields about 1,012 neutrons per second (n/s), which is a much higher neutron yield than Am-Be sources. A typical Cf-252 source with an activity rating of 20 mCi emits about 108 n/s compared to 16 CI from Am-Be. This fission reaction has a half-life of 2.6 years compared to 430 years for Am-Be, which means that the neutron output of the source drops by a factor of two every 2.6 years. However this reduces the useful life of the source to somewhere between 5 and 10 years. Although higher-output Cf-252 sources can be obtained to lengthen the useful life of the source, cost and safety may be issues.

To prove the effectiveness of the new source, it was decided to run the Cf-252 back-to-back with the Am-Be source. By running both sources in the same bottomhole assembly (BHA), the quality of the Cf 252 could be validated by comparing the back-to-back measurements generated from both Cf-252 and Am-Be sources. The data was compared while wiping in, wiping out, and drilling. The Cf-252 source tool was placed on the top of all other tools in the BHA in recorded memory mode. The data acquired while drilling showed a slight separation between Cf-252 porosity readings and the readings from the tool using an Am-Be. This is due to the varying formation exposure time experienced by the tool with the Cf-252 source, an invasion effect observed due to the slow rate of penetration (ROP). There is also much closer agreement between the two neutron measurements while wiping out.

This case study demonstrates the value of using a low radiation energy Cf-252 source compared to using high radiation energy Am-Be source when drilling in a depleted reservoir. A lost-in-hole Am-Be source had to be fished, which prevented the operator from plugging the well. However, using a Cf-252 source would minimize such liabilities, making the plug back procedure easier with significant savings in time and money.