Abstract

Several wells with bottom hole temperatures in excess of 400F have been drilled in South Australia. Two early wells were drilled with Lignite-Lignosulfonate fluids and two recent wells used a newly introduced high temperature polymer system. A brief comparison of these high temperature wells has been made. The effects of temperature on the performance and properties of the polymer system are examined and the need for further polymer system are examined and the need for further improvements in the area of high temperature drilling fluids is also considered.

INTRODUCTION AND GEOLOGY

The Cooper-Eromanga Basin in Australia comprises sediments ranging in age from Cretaceous to the Pre-Permian basement. From surface to the "Transition Pre-Permian basement. From surface to the "Transition Beds" at 5300 ft, the formations are mostly mud-making silts and clays. Below the transition zone, the formations are sandier in nature with interbeds of sand and silt and occasional coals. The boundary between the Cooper and Eromanga Basins is in the Early Jurassic-Upper Triassic period at 7200 to 7300 ft. The Cooper Basin is Permo-Triassic while the greater Eromanga Basin is Jurassic-Cretaceous. Temperature gradients in the area range from 1.7F/100 ft to 3.1F/100 ft, however most wells are less than 10,000 ft deep and thus the higher temperature ranges are not usually encountered. The four high temperature wells discussed have been drilled in the Moomba Block 2A of Petroleum Exploration Leases 5 and 6 in South Australia. A locality map is given in Figure 1. These hot wells have been drilled with several aims:

  1. As exploratory holes

  2. As part of the operator's accelerated gas development project Though the wells only have a temperature gradient of 2.8F/100 ft, they have been drilled deeper to 12,000 ft, at which point bottom hole temperatures of 400F have been approached.

A general stratigraphic column of the wells is given in Table 1.

CASE HISTORY NO. 1

The first well was drilled in 1971 to a depth of 11,974 ft and the bottom hole temperature at TD was 413F. Pressure gradients of up to 0.73 psi/ft were encountered. Drilling and mud data for this well are given in Table 2.

Top Interval - 17 1/2" Hole: 0 to 1172 ft

This interval was drilled with a Gel spud mud, but due to the presence of hard duracrust it took 4 days to reach casing point of 1172 ft. Difficulty was experienced in running in 13 3/8" casing, and it took 8 days to finally cement the casing in place.

Intermediate Interval - 12 1/4" Hole: 1172 to 6211 ft

This section was drilled with an 8 1/2" bit using Water-Native Clay mud through mud-making silts and clays into the "Transition Beds" at 5600 ft. The system was then mudded up to a Lignosulfonate fluid and drilling continued with a 9.7 ppg mud weight into the Toolachee Formation. At 8734 ft the mud weight had to be increased to 10.3 ppg to control gas within the coals and sands. Drilling was suspended at 8790 ft and the hole was opened to 12 1/4" to 6211 ft. 9 5/8" casing was then cemented in place.

Production Interval - 8 1/2" Hole: 6211 to 11,974 ft Production Interval - 8 1/2" Hole: 6211 to 11,974 ft Drilling continued with a 10.0 ppg Lignosulfonate mud. The mud weight was slowly increased to 10.8 ppg at 9261 ft, up to which point apparently normal ppg at 9261 ft, up to which point apparently normal pressure gradients were encountered. When running pressure gradients were encountered. When running in for DST No. 5, part of the test assembly was lost, but was recovered on the first attempt with an overshot. Drilling continued to 10,107 ft when the weight had to be increased to 11.2 ppg to contain high pressured gas in the Epsilon Formation. An 80 barrel gas kick occurred which was finally killed by increasing the mud weight to 13.3 ppg. At 10,172 ft the mud weight had to be increased again to 14.6 ppg. DST No. 10 at 10,241 ft indicated a pressure ppg. DST No. 10 at 10,241 ft indicated a pressure gradient of 0.73 psi/ft and a bottom hole temperature of 360F. Additions of Lignite for high temperature fluid loss control were commenced with a Lignite : Lignosulfonate ratio of 1:1. 5% by volume of diesel oil was also added to improve the high temperature properties of the mud. Drilling continued to T.D. at 11,974 ft with a 14.6 - 14.8 ppg mud weight. ppg mud weight. Severe flocculation and dehydration occurred below 10,400 ft. Chemical treatment was not effective, necessitating large water dilutions which led to a high barite consumption to maintain mud weight. Logging was completed with difficulty and most tools did not reach bottom, probably due to downhole mud gelation. The well was then plugged and abandoned. A total of 12 DST tests and 4 cores were taken. The mud cost based on 1984 prices was $207,277 and the well took 107 drilling days and 127 total days. The maximum mud weight used was 14.8 ppg and the maximum flowline temperature encountered was 180F. The estimated bottom hole temperature was 413F.

CASE HISTORY NO. 2

The second well was drilled in 1978 to a depth of 12,685 ft and bottom hole temperature at TD was estimated at 375F. The well was apparently normally pressured to 10,500 ft. A gradient of 0.65 psi/ft pressured to 10,500 ft. A gradient of 0.65 psi/ft was encountered at 10,718 ft and at 12,462 ft the gradient had increased to 0.73 psi/ft. Drilling and mud data for this well are given in Table 3.

Top Interval - 17 1/2" Hole: 0 to 852 ft The well was spudded with a Gel-spud mud, and drilled to 832 ft. 13 3/8" casing was subsequently run and cemented.

Intermediate Interval - 12 1/4" Hole: 852 to 6600 ft

A Water-Native Clay mud was used to 3300 ft, and was then mudded up to a lightly dispersed Lignite- Lignosulfonate mud. Before reaching the Transition Zone, the mud weight was increased to 10.0 ppg with drilled solids. The viscosity ranged from 40-50 sec/qt. At 5266 ft drilling was halted for 7 days due to rig repairs. Drilling then continued and tight hole was encountered at 5600 ft which required extensive reaming to bottom. To combat hole wash-out, the viscosity was increased to 45-50 sec/qt to casing point of 6600 ft. After logging, 9 5/8" casing was run and cemented.

Production Interval - 8 1/2" Hole: 6600 to 12,685 ft Production Interval - 8 1/2" Hole: 6600 to 12,685 ft Drilling continued with mud from the previous interval and properties were improved as drilling progressed. progressed.

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