Acidizing deviated wells is a technical challenge because the primary cementing is not always of high quality. Large communicating channels can develop behind the casing. Under such conditions, conventional selective acidizing is ineffective since the acid leaks off into these channels following the path of the least resistance.
A technique has been developed to effectively acidize such wells. The channels were temporarily sealed using gelled diesel and salt water. The acid spotted subsequently could no longer leak off into these channels; instead, it was diverted into the formation.
Using this technique, an injection well in the Norman Wells field was successfully acidized. Excellent vertical conformance resulted, and high daily injection rates were also realized.
Located approximately 80km south of the Arctic circle In the Northwest Territories, the Norman Wells field currently produces 5200 m3 / day of 38 ° API sweet oil. The field is currently waterflooded. A combined total of 8800 m3/day fresh and produced waters is being injected into 155 injection wells. It is anticipated that the injection rate will be increased in the near future to further enhance oil production. The current water injection strategies are (a) to maintain a target reservoir pressure throughout the pool (good areal sweep), and (b) to improve vertical conformance along the 100 m thick pay zone (good vertical sweep).
Various workover methods have been performed to achieve these objectives. They include rigorous flowbacks, acid washes of the existing perforations, perforating additional intervals as well as re-shooting over the existing perforations, and selective acidization. An analysis of more than 50 such workovers has indicated that selective acidizing is probably the most effective method to meet the workover objectives. However, some problems were encountered using this technique in the field.
Many of the wells are highly deviated, and quality primary cement Jobs were not always obtained. (Also, the cement may have deteriorated over the years.) Under these conditions, selective acidizing is ineffective because the acid preferentially leaks off into these channels and can not be squeezed off into the target zones. The nature of the reservoir (carbonate) further aggravates the problem, as the acid can enlarge the channels by dissolving the rock around them.
To ensure effective acidizing, a technique was designed that would temporarily seal the communicating channels and divert the acid into the formation. This paper describes the technique and the field experiment that evaluates its effectiveness.
Lying beneath the Mackenzie River bed, the Norman Wells field is pooled in a 25 km long by B km wide Kee Scarp limestone reef. The reef is of Middle Devonian age, roughly time equivalent to the Swan Hills formation reefs of west-central Alberta. The 100 m thick formation is divided into four markers or cycles which are characteristic of sea level controlled reef growth. These cycles are further sub-divided into a total of ten different horizons. Table 1 describes some of the key reservoir characteristics.
The field was extensively expanded in the eighties. The current production strategy is to maximize production from the entire pay zone.